doc: Use proper tags in pg_overexplain documentation.

doc: Use proper tags in pg_overexplain documentation.

The pg_overexplain documentation previously used the <literal> tag for
some file names, struct names, and commands. Update the markup to
use the more appropriate tags: <filename>, <structname>, and <command>.

Backpatch to v18, where pg_overexplain was introduced.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Shixin Wang <wang-shi-xin@outlook.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwEyYUzz0LjBV_fMcdwU3wgmu0NCoT+JJiozPa8DG6eeog@mail.gmail.com
Backpatch-through: 18