commitctx: document _filecommit input and output

commitctx: document _filecommit input and output

This is the beginning of a larger refactoring/cleanup of the commitctx code to
clarify and augment the logic gathering metadata useful for copy tracing. The
current code is a tad too long and entangled to make such update easy. We start
with easy and small cleanup.

Differential Revision: https://phab.mercurial-scm.org/D8699

--- a/mercurial/localrepo.py	Tue Jul 07 21:57:46 2020 +0200
+++ b/mercurial/localrepo.py	Mon Jul 06 18:52:19 2020 +0200
@@ -2786,6 +2786,22 @@
     ):
         """
         commit an individual file as part of a larger transaction
+
+        input:
+
+            fctx:       a file context with the content we are trying to commit
+            manifest1:  manifest of changeset first parent
+            manifest2:  manifest of changeset second parent
+            linkrev:    revision number of the changeset being created
+            tr:         current transation
+            changelist: list of file being changed (modified inplace)
+            individual: boolean, set to False to skip storing the copy data
+                        (only used by the Google specific feature of using
+                        changeset extra as copy source of truth).
+
+        output:
+
+            The resulting filenode
         """
 
         fname = fctx.path()