streamclone: rename and document maybeperformstreamclone()

Upcoming patches will introduce bundle2 based streaming clones. Add
"legacy" to the function name and add a docstring clarifying the intent of
the function.

mercurial/exchange.py

@@ -964,7 +964,7 b' def pull(repo, remote, heads=None, force'
     lock = pullop.repo.lock()
     try:
         pullop.trmanager = transactionmanager(repo, 'pull', remote.url())
-        streamclone.maybeperformstreamclone(pullop)
+        streamclone.maybeperformlegacystreamclone(pullop)
         _pulldiscovery(pullop)
         if _canusebundle2(pullop):
             _pullbundle2(pullop)

mercurial/streamclone.py

@@ -66,7 +66,15 b' def canperformstreamclone(repo, remote, '
 
     return True, requirements
 
-def maybeperformstreamclone(pullop):
+def maybeperformlegacystreamclone(pullop):
+    """Possibly perform a legacy stream clone operation.
+
+    Legacy stream clones are performed as part of pull but before all other
+    operations.
+
+    A legacy stream clone will not be performed if a bundle2 stream clone is
+    supported.
+    """
     repo = pullop.repo
     remote = pullop.remote