Include Hash#size in the examples

Both methods Hash#length and Hash#size share the same source code in
Ruby, but they also share the same documentation. Now when you look at
the documentation of Hash#size you only see examples for Hash#length,
which is confusing. This commit includes Hash#size in the examples and
also remarks that both methods are equivalent to each other.

Co-authored-by: Alberto Almagro <alberto.almagro@rakuten.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64081 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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