Document why `RE2` objects are neither copyable nor movable.
Document why `RE2` objects are neither copyable nor movable.

Change-Id: Icf86a2e12f4f860bd6406b1326324d987659413f
Reviewed-on: https://code-review.googlesource.com/c/re2/+/60410
Reviewed-by: Perry Lorier <d2756efe887c28699f4587ef278926ee3b3bfae5@google.com>
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

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