scmutil: improve documentation of writereporequirements()

This makes it easier to understand the difference between `writerequires()`
and `writereporequirements()`.

Differential Revision: https://phab.mercurial-scm.org/D9568

mercurial/scmutil.py

@@ -1573,7 +1573,12 b' def istreemanifest(repo):'
 
 
 def writereporequirements(repo, requirements=None):
-    """ writes requirements for the repo to .hg/requires """
+    """writes requirements for the repo
+
+    Requirements are written to .hg/requires and .hg/store/requires based
+    on whether share-safe mode is enabled and which requirements are wdir
+    requirements and which are store requirements
+    """
     if requirements:
         repo.requirements = requirements
     wcreq, storereq = filterrequirements(repo.requirements)