docs-xml: Add a section about weak crypto in testparm manpage

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14583Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>

Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Fri Nov 27 13:48:20 UTC 2020 on sn-devel-184

docs-xml/manpages/testparm.1.xml

@@ -171,6 +171,15 @@
 	errors and warnings if the file did not load. If the file was 
 	loaded OK, the program then dumps all known service details 
 	to stdout. </para>
+
+	<para>For certain use cases, SMB protocol requires use of
+	cryptographic algorithms which are known to be weak and already
+	broken. DES and ARCFOUR (RC4) ciphers and the SHA1 and MD5 hash
+	algorithms are considered weak but they are required for backward
+	compatibility. The testparm utility shows whether the Samba tools
+	will fall back to these weak crypto algorithms if it is not possible
+	to use strong cryptography by default.
+	In FIPS mode weak crypto cannot be enabled.</para>
 </refsect1>