Update Consume's method comment re: empty matches.

Consume's behavior is explained in re2.h:144, but the method comment
contradicted that explanation.

Change-Id: I92fee81141ead46dd7015b85d4e370160e488180
Reviewed-on: https://code-review.googlesource.com/c/re2/+/39810
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed