Small documentation adjustments for new/updated features (#15634)

* Document Range#to_set

* Update Thread#raise and Fiber#raise signatures and docs

* Add reference to String#strip to character_selectors.rdoc

* Update *nil docs when calling methods

* Enhance Array#find and #rfind docs

* Add a notice to Kernel#raise about cause:

1 files changed, 3 insertions, 0 deletions

diff --git a/eval.c b/eval.c
index 0c80872bee..deadd5dd64 100644
--- a/eval.c
+++ b/eval.c
@@ -922,6 +922,9 @@ rb_f_raise(int argc, VALUE *argv)
  *  With argument +exception+ not given,
  *  argument +message+ and keyword argument +cause+ may be given,
  *  but argument +backtrace+ may not be given.
+ *
+ *  +cause+ can not be given as an only argument.
+ *
  */
 
 static VALUE