dpkg-source(1): more explanations for dpkg-source --commit

Clarify the conditions under which it's possible to pass an explicit patch
file to dpkg-source --commit (for "3.0 (quilt)" source packages). This
option has a very narrow purpose: it's mainly to avoid having to
regenerate the patch which can be a costly operation for large packages
and thus assumes several things that one should not forget when using it.

debian/changelog

@@ -68,6 +68,8 @@ dpkg (1.16.2) UNRELEASED; urgency=low
     Closes: #651993
   * Fix dpkg-source --commit on "3.0 (quilt)" when an explicit patch file
     is given with a relative filename. Closes: #652414
+  * Further clarify in dpkg-source(1) the conditions under which it's possible
+    to pass an explicit patch file to dpkg-source --commit.
 
   [ Jonathan Nieder ]
   * Bump po4a version in Build-Depends to 0.41, since earlier versions do

man/dpkg-source.1

@@ -483,11 +483,15 @@ Generates a patch corresponding to the local changes that are not managed
 by the quilt patch system and integrates it in the patch system under
 the name \fIpatch-name\fP. If the name is missing, it will be asked
 interactively. If \fIpatch-file\fP is given, it is used as the patch
-corresponding to the local changes to integrate. This is mainly useful
-after a build failure that pre-generated this file. Once integrated, an
+corresponding to the local changes to integrate. Once integrated, an
 editor is launched so that you can edit the meta-information in the patch
 header.
 
+Passing \fIpatch-file\fP is mainly useful after a build failure that
+pre-generated this file, and on this ground the given file is removed
+after integration. Note also that the changes contained in the patch file
+must already be applied on the tree and that the files modified by the
+patch must not have supplementary unrecorded changes.
 .PP
 .B Build options
 .TP