[gcc r9-9764] doc: improve -fsanitize=undefined description

[gcc r9-9764] doc: improve -fsanitize=undefined description

[Richard Sandiford]

https://gcc.gnu.org/g:48c71dbb0d95396a7ef37c97006535751d523f22

commit r9-9764-g48c71dbb0d95396a7ef37c97006535751d523f22
Author: Diane Meirowitz <diane.meirowitz@oracle.com>
Date:   Wed Sep 15 21:03:00 2021 +0000

    doc: improve -fsanitize=undefined description
    
    gcc/ChangeLog:
            * doc/invoke.texi: Add link to UndefinedBehaviorSanitizer
            documentation, mention UBSAN_OPTIONS, similar to what is done
            for AddressSanitizer.
    
    (cherry picked from commit 1c0a83eff7bb5b1db997a9726ae6542aec893baa)

Diff:
---
 gcc/doc/invoke.texi | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 7226c9cc4af..977fe4574ec 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -12484,7 +12484,8 @@ The option cannot be combined with @option{-fsanitize=thread}.
 @opindex fsanitize=undefined
 Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector.
 Various computations are instrumented to detect undefined behavior
-at runtime.  Current suboptions are:
+at runtime.  See @uref{https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html} for more details.   The run-time behavior can be influenced using the
+@env{UBSAN_OPTIONS} environment variable.  Current suboptions are:
 
 @table @gcctabopt