Add a clarifying comment about case folding.

Fixes #262.

Change-Id: I31a87f32fb780e40a7077a88168467d1c817fd3f
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57230
Reviewed-by: Paul Wankadia <junyer@google.com>
(cherry picked from commit 2cd55ab74d95cbad37c8be5b4fc5a814b0184482)
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57231