libstdc++: Improve doxygen comments in <bits/stl_function.h>

Add notes about deprecation and modern replacements. Fix bogus
"memory_adaptors" group name. Use markdown for formatting.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/bits/stl_function.h: Improve doxygen comments.