docs: Improve alt text on existing rewrite diagrams for accessibility

docs: Improve alt text on existing rewrite diagrams for accessibility

Replace vague or duplicated alt attributes with descriptive text that
conveys the content of each diagram to screen reader users:
- rewrite_process_uri.png: describes per-rule control flow
- rewrite_backreferences.png: describes \$1-\$9 and %1-%9 flow
- syntax_rewriterule.png: describes three-component syntax
- syntax_rewritecond.png: describes TestString and CondPattern components

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1935429 13f79535-47bb-0310-9956-ffa450edef68