lto/95190 - amend -flto docs for diagnostic option handling

This documents new GCC 10 behavior on diagnostic options and -flto.

2020-05-22  Richard Biener  <rguenther@suse.de>

	PR lto/95190
	* doc/invoke.texi (flto): Document behavior of diagnostic
	options.

(cherry picked from commit ab7eca92926fdc1da880120c116a1832fce56a29)

gcc/ChangeLog

+2020-05-22  Richard Biener  <rguenther@suse.de>
+
+	PR lto/95190
+	* doc/invoke.texi (flto): Document behavior of diagnostic
+	options.
+
 2020-05-21  Uroš Bizjak  <ubizjak@gmail.com>
 
 	PR target/95169

gcc/doc/invoke.texi

@@ -11204,6 +11204,14 @@ conflicting translation units.  Specifically
 precedence; and for example @option{-ffp-contract=off} takes precedence
 over @option{-ffp-contract=fast}.  You can override them at link time.
 
+Diagnostic options such as @option{-Wstringop-overflow} are passed
+through to the link stage and their setting matches that of the
+compile-step at function granularity.  Note that this matters only
+for diagnostics emitted during optimization.  Note that code
+transforms such as inlining can lead to warnings being enabled
+or disabled for regions if code not consistent with the setting
+at compile time.
+
 When you need to pass options to the assembler via @option{-Wa} or
 @option{-Xassembler} make sure to either compile such translation
 units with @option{-fno-lto} or consistently use the same assembler