Update Consume's method comment re: empty matches.
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 <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2