docs: add note about regex

[caleb15]

to prevent people from running into #3436

What this PR does / why we need it:

• The |~ operator is a partial match. One would intuitively expect that =~ would behave the same way, but it is in fact a exact match. It's important to point that out.
• Many log lines have newlines in them (at least in my case), so it's important to remind users that . does not match newlines by default.

Which issue(s) this PR fixes:
Fixes #3436

Special notes for your reviewer:

Checklist

• Documentation added
• [N/A] Tests updated

[CLAassistant]

All committers have signed the CLA.

[owen-d]

Nice digging to find this :).