Follow-up to r1597962:
Update commentary on functions that use the SVN__FORCE_INLINE macro.
No functional change.

* subversion/libsvn_fs_fs/index.c
  (packed_stream_read): Explain our usage of SVN__PREVENT_INLINE in
                        more detail, using the correct function names.
  (packed_stream_get): Update commentary on SVN__FORCE_INLINE.

* subversion/libsvn_fs_x/index.c
  (packed_stream_read,
   packed_stream_get): As above for FSFS.

* subversion/libsvn_ra_svn/marshal.c
  (readbuf_getchar): Say why we use SVN__FORCE_INLINE here.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1598273 13f79535-47bb-0310-9956-ffa450edef68