docs: update search documentation

Created by: rvantonder

Adds a more complete description of search syntax (specifically for search patterns) and interpretation. This will help avoid issues like #7604 (closed) and #6490 (closed). These changes will also become more important with the addition of a content: filter and AND/OR expressions down the line.

I do wonder if the search pattern section should come after the keyword section. Your thoughts here.

Detailed changes:

• Add a search pattern syntax section that clarifies:
  • case sensitivity for patterns
  • literal search ordering and current quotes interpretation
  • that we interpolate .*? in regex by default
  • that the above can be avoided with /.../ syntax or escaping in regex mode
  • that newline search is possible in regex mode
  • the structural search toggle and need for quoting for structural search (need to wait on #7960 (closed) for this)
  • interpretation of quoting in regexp search
  • interpretation of quoting in structural search
• move the note about default search settings after literal search, and make it slightly shorter
• removes the 'any-string' and '"any-string"' rows from the keyword section in favor of the search pattern syntax section above. The current description for quoting in the keyword table isn't really accurate, and there isn't really a "keyword" here.
• make the keyword ordering for search literal, regexp, structural and move it to the end since visible toggles take precedence now