I think it'd be good to have all superceded functions appear in the
deprecated chapter of the manual.  It can say what they do, for the
benefit of anyone seeing them in old code, and can explain what
they've been replaced by or what was wrong with them.  As a bit of a
start:

	* deprecated.texi: Add substring-move-left! and substring-move-right!.


Deprecated
**********

The features and functions described in this chapter are considered
obsolete and should not be used by new programs.  They're retained for
compatibility with past versions of Guile.

 - Scheme Procedure: substring-move-left! str1 start1 end1 str2 start2
 - Scheme Procedure: substring-move-right! str1 start1 end1 str2 start2
     These functions are aliases for `substring-move!' and calls to
     them can simply be replaced by `substring-move!' (*note String
     Modification::.)

     In the past, if STR1 and STR2 were the same string, it was
     necessary to call `substring-move-left!' if the move was to the
     left (START2 < START1), or call `substring-move-right!' if the
     move was to the right (START2 > START1).  `substring-move!' is able
     to do both.