header_rewrite: set-destination path doc and warning updates

[bneradt]

This patch updates the docs to indicate that operator string parameters containing any of the operands should be quoted. It also adds a warning check for strings starting with any of these (<, >, =, /).

Fixes: #12173

[bneradt]

Thank you for the thoughtful doc comments, @mlibbey . @zwoop provided feedback as well that the intention is for the tokenization issues to be address via quoting the path string. I've updated the docs to make this quoting expectation more clear too (in addition to the expectation that the path string shouldn't start with an initial /).

Let me know if you have other observations after these updates.

[bneradt]

I'm going to close this out. Hopefully this is less needed with the hdrw4u changes.