Maybe this should be documented in Emacs Lisp Reference or in other
suitable info, probably Byte Compiler Users Guide. Also other
byte-compiler optimizations should be documented.
Actually, I don't think optimizations should be particularly mentioned.
The interesting part of optimizing (featurep 'xemacs) is that it
eliminates
spurious warnings. There are other ways to get the same effect, such as
what is done with (if (fboundp 'foo) ...) where the test is not optimized
away, but where warnings are selectively prevented.
I think a chapter on "eliminating warnings" is in order to document
the use of
(featurep 'xemacs), (fboundp 'foo), (defvar foo), (with-no-warning
...), ...