man: Document that timestamps are reset for format 3.0 (quilt) too

Prompted-by: Niko Tyni <ntyni@debian.org> (in #759404)

debian/changelog

@@ -121,6 +121,8 @@ dpkg (1.18.0) UNRELEASED; urgency=low
     dselect(1) man pages.
   * Properly terminate a bold marking in dpkg(1).
   * Document in man pages the dpkg version when new features were introduced.
+  * Document that timestamps are reset for files patched with source format
+    “3.0 (quilt)” too.
 
   [ Raphaël Hertzog ]
   * Drop myself from Uploaders.

man/dpkg-source.1

@@ -476,6 +476,10 @@ options and always expect patches that can be applied with the \fB\-p1\fP
 option of \fBpatch\fP. It will thus emit a warning when it encounters
 such options, and the build is likely to fail.
 .PP
+The timestamp of all patched files is reset to the extraction time of
+the source package (this avoids timestamp skews leading to problems
+when autogenerated files are patched).
+.PP
 Contrary to \fBquilt\fP's default behaviour, patches are expected to apply
 without any fuzz. When that is not the case, you should refresh such
 patches with \fBquilt\fP, or \fBdpkg\-source\fP will error out while