dpkg-buildflags(1): Move generic feature area details to the section top

These are valid for any future feature area, so share the information to
avoid having to repeat it on each subsection.

debian/changelog

@@ -33,6 +33,7 @@ dpkg (1.17.14) UNRELEASED; urgency=low
   * Man pages:
     - Add a mention of packaging helpers support in dpkg-maintscript-helper(1).
       Closes: #759754
+    - Move dpkg-buildflags(1) generic feature area details to the section top.
   * Add new deps_iterate() function to Dpkg::Deps. Closes: #759747
     Based on a patch by Dima Kogan <dima@secretsauce.net>.
   * Add support for relative symlinks to dpkg-maintscript-helper symlink_to_dir

man/dpkg-buildflags.1

@@ -196,26 +196,28 @@ New flags might be added in the future if the need arises (for example
 to support other languages).
 .
 .SH FEATURE AREAS
-.SS Hardening
-Several compile-time options (detailed below) can be used to help harden
-a resulting binary against memory corruption attacks, or provide
-additional warning messages during compilation. Except as noted below,
-these are enabled by default for architectures that support them.
 .P
-Each hardening feature can be enabled and disabled in the
-\fBDEB_BUILD_MAINT_OPTIONS\fP environment variable's \fBhardening\fP
-value with the "+" and "\-" modifier. For example, to enable the
-"pie" feature and disable the "fortify" feature you can do this
-in \fBdebian/rules\fP:
+Each area feature can be enabled and disabled in the
+\fBDEB_BUILD_MAINT_OPTIONS\fP environment variable's area value with the
+"+" and "\-" modifier.
+For example, to enable the \fBhardening\fP "pie" feature and disable the
+"fortify" feature you can do this in \fBdebian/rules\fP:
 .P
   export DEB_BUILD_MAINT_OPTIONS=hardening=+pie,\-fortify
 .P
-The special feature \fBall\fP can be used to enable or disable all
-hardening features at the same time. Thus disabling everything and
-enabling only "format" and "fortify" can be achieved with:
+The special feature \fBall\fP (valid in any area) can be used to enable or
+disable all area features at the same time.
+Thus disabling everything in the \fBhardening\fP area and enabling only
+"format" and "fortify" can be achieved with:
 .P
   export DEB_BUILD_MAINT_OPTIONS=hardening=\-all,+format,+fortify
 .
+.SS Hardening
+Several compile-time options (detailed below) can be used to help harden
+a resulting binary against memory corruption attacks, or provide
+additional warning messages during compilation.
+Except as noted below, these are enabled by default for architectures
+that support them.
 .TP
 .B format
 This setting (enabled by default) adds