misc: update descriptions about removed file for filectxfn

misc: update descriptions about removed file for filectxfn

Since 650b5b6e75ed, filectxfn for memctx should return None for
removed file instead of raising IOError.

--- a/hgext/largefiles/lfcommands.py	Mon Mar 20 17:58:44 2017 -0400
+++ b/hgext/largefiles/lfcommands.py	Fri Mar 24 22:13:23 2017 +0900
@@ -246,7 +246,7 @@
     def getfilectx(repo, memctx, f):
         if lfutil.isstandin(f):
             # if the file isn't in the manifest then it was removed
-            # or renamed, raise IOError to indicate this
+            # or renamed, return None to indicate this
             srcfname = lfutil.splitstandin(f)
             try:
                 fctx = ctx.filectx(srcfname)

--- a/mercurial/context.py	Mon Mar 20 17:58:44 2017 -0400
+++ b/mercurial/context.py	Fri Mar 24 22:13:23 2017 +0900
@@ -1864,8 +1864,8 @@
     commit function for every file in 'files', but calls order is
     undefined. If the file is available in the revision being
     committed (updated or added), filectxfn returns a memfilectx
-    object. If the file was removed, filectxfn raises an
-    IOError. Moved files are represented by marking the source file
+    object. If the file was removed, filectxfn return None for recent
+    Mercurial. Moved files are represented by marking the source file
     removed and the new file added with copy information (see
     memfilectx).