Bsearch doc for Array and Range (#4838)

This PR creates doc/bsearch.rdoc to provide common documentation for bsearch in Array and Range.

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+== Binary Searching
+
+A few Ruby methods support binary searching in a collection:
+
+Array#bsearch:: Returns an element selected via a binary search
+                as determined by a given block.
+Array#bsearch_index:: Returns the index of an element selected via a binary search
+                      as determined by a given block.
+Range#bsearch:: Returns an element selected via a binary search
+                as determined by a given block.
+
+Each of these methods returns an enumerator if no block is given.
+
+Given a block, each of these methods returns an element (or element index) from +self+
+as determined by a binary search.
+The search finds an element of +self+ which meets
+the given condition in <tt>O(log n)</tt> operations, where +n+ is the count of elements.
++self+ should be sorted, but this is not checked.
+
+There are two search modes:
+
+Find-minimum mode:: method +bsearch+ returns the first element for which
+                    the block returns +true+;
+                    the block must return +true+ or +false+.
+Find-any mode:: method +bsearch+ some element, if any, for which
+                the block returns zero.
+                the block must return a numeric value.
+
+The block should not mix the modes by sometimes returning +true+ or +false+
+and other times returning a numeric value, but this is not checked.
+
+<b>Find-Minimum Mode</b>
+
+In find-minimum mode, the block must return +true+ or +false+.
+The further requirement (though not checked) is that
+there are no indexes +i+ and +j+ such that:
+
+- <tt>0 <= i < j <= self.size</tt>.
+- The block returns +true+ for <tt>self[i]</tt> and +false+ for <tt>self[j]</tt>.
+
+Less formally: the block is such that all +false+-evaluating elements
+precede all +true+-evaluating elements.
+
+In find-minimum mode, method +bsearch+ returns the first element
+for which the block returns +true+.
+
+Examples:
+
+  a = [0, 4, 7, 10, 12]
+  a.bsearch {|x| x >= 4 } # => 4
+  a.bsearch {|x| x >= 6 } # => 7
+  a.bsearch {|x| x >= -1 } # => 0
+  a.bsearch {|x| x >= 100 } # => nil
+
+  r = (0...a.size)
+  r.bsearch {|i| a[i] >= 4 } #=> 1
+  r.bsearch {|i| a[i] >= 6 } #=> 2
+  r.bsearch {|i| a[i] >= 8 } #=> 3
+  r.bsearch {|i| a[i] >= 100 } #=> nil
+  r = (0.0...Float::INFINITY)
+  r.bsearch {|x| Math.log(x) >= 0 } #=> 1.0
+
+These blocks make sense in find-minimum mode:
+
+  a = [0, 4, 7, 10, 12]
+  a.map {|x| x >= 4 } # => [false, true, true, true, true]
+  a.map {|x| x >= 6 } # => [false, false, true, true, true]
+  a.map {|x| x >= -1 } # => [true, true, true, true, true]
+  a.map {|x| x >= 100 } # => [false, false, false, false, false]
+
+This would not make sense:
+
+  a.map {|x| x == 7 } # => [false, false, true, false, false]
+
+<b>Find-Any Mode</b>
+
+In find-any mode, the block must return a numeric value.
+The further requirement (though not checked) is that
+there are no indexes +i+ and +j+ such that:
+
+- <tt>0 <= i < j <= self.size</tt>.
+- The block returns a negative value for <tt>self[i]</tt>
+  and a positive value for <tt>self[j]</tt>.
+- The block returns a negative value for <tt>self[i]</tt> and zero <tt>self[j]</tt>.
+- The block returns zero for <tt>self[i]</tt> and a positive value for <tt>self[j]</tt>.
+
+Less formally: the block is such that:
+
+- All positive-evaluating elements precede all zero-evaluating elements.
+- All positive-evaluating elements precede all negative-evaluating elements.
+- All zero-evaluating elements precede all negative-evaluating elements.
+
+In find-any mode, method +bsearch+ returns some element
+for which the block returns zero, or +nil+ if no such element is found.
+
+Examples:
+
+  a = [0, 4, 7, 10, 12]
+  a.bsearch {|element| 7 <=> element } # => 7
+  a.bsearch {|element| -1 <=> element } # => nil
+  a.bsearch {|element| 5 <=> element } # => nil
+  a.bsearch {|element| 15 <=> element } # => nil
+
+  a = [0, 100, 100, 100, 200]
+  r = (0..4)
+  r.bsearch {|i| 100 - a[i] } #=> 1, 2 or 3
+  r.bsearch {|i| 300 - a[i] } #=> nil
+  r.bsearch {|i|  50 - a[i] } #=> nil
+
+These blocks make sense in find-any mode:
+
+  a = [0, 4, 7, 10, 12]
+  a.map {|element| 7 <=> element } # => [1, 1, 0, -1, -1]
+  a.map {|element| -1 <=> element } # => [-1, -1, -1, -1, -1]
+  a.map {|element| 5 <=> element } # => [1, 1, -1, -1, -1]
+  a.map {|element| 15 <=> element } # => [1, 1, 1, 1, 1]
+
+This would not make sense:
+
+  a.map {|element| element <=> 7 } # => [-1, -1, 0, 1, 1]