Restore RawVec::reserve* documentation

Restore RawVec::reserve* documentation

When the RawVec::try_reserve* methods were added, they took the place of
the ::reserve* methods in the source file, and new ::reserve* methods
wrapping the new try_reserve* methods were created. But the
documentation didn't move along, such that:
 - reserve_* methods are barely documented.
 - try_reserve_* methods have unmodified documentation from reserve_*,
   such that their documentation indicate they are panicking/aborting.

This moves the documentation back to the right methods, with a
placeholder documentation for the try_reserve* methods.