Document OPENSSL_VERSION_TEXT macro

This commit documents the OPENSSL_VERSION_TEXT which is currently
missing in the man page.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7301)

(cherry picked from commit 7c694957)