dev-manual: improve descriptions of 'bitbake -S printdiff'

Try to particularly emphasize that it can be used to find
out why something rebuilds when it shouldn't.

(From yocto-docs rev: 05d08b0bbaef760157c8d35a78d7405bc5ffce55)

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/dev-manual/building.rst

@@ -775,10 +775,9 @@ your tunings to best consider build times and package feed maintenance.
       in the script for information on how to use the tool.
 
    -  *BitBake's "-S printdiff" Option:* Using this option causes
-      BitBake to try to establish the closest signature match it can
-      (e.g. in the shared state cache) and then run ``bitbake-diffsigs``
-      over the matches to determine the stamps and delta where these two
-      stamp trees diverge.
+      BitBake to try to establish the most recent signature match
+      (e.g. in the shared state cache) and then compare matched signatures
+      to determine the stamps and delta where these two stamp trees diverge.
 
 Building Software from an External Source
 =========================================

documentation/dev-manual/debugging.rst

@@ -339,7 +339,10 @@ BitBake has determined by doing the following:
    :term:`BB_BASEHASH_IGNORE_VARS`
    information.
 
-There is also a ``bitbake-diffsigs`` command for comparing two
+Debugging signature construction and unexpected task executions
+===============================================================
+
+There is a ``bitbake-diffsigs`` command for comparing two
 ``siginfo`` or ``sigdata`` files. This command can be helpful when
 trying to figure out what changed between two versions of a task. If you
 call ``bitbake-diffsigs`` with just one file, the command behaves like
@@ -356,8 +359,12 @@ BitBake command-line options::
 .. note::
 
    Two common values for `SIGNATURE_HANDLER` are "none" and "printdiff", which
-   dump only the signature or compare the dumped signature with the cached one,
-   respectively.
+   dump only the signature or compare the dumped signature with the most recent one,
+   respectively. "printdiff" will try to establish the most recent
+   signature match (e.g. in the sstate cache) and then
+   compare the matched signatures to determine the stamps and delta
+   where these two stamp trees diverge. This can be used to determine why
+   tasks need to be re-run in situations where that is not expected.
 
 Using BitBake with either of these options causes BitBake to dump out
 ``sigdata`` files in the ``stamps`` directory for every task it would