Document SEARCH_MAX_DEPTH option for SAST analyzers

What does this MR do?

The SEARCH_MAX_DEPTH variable defaults to 4 as discussed and decided in #209747 (comment 354248167).

It's important to document this variable because the rules to trigger an analyzer use glob patterns (example), which are not depth limited. This causes sast jobs to fail if a source file is in a directory depth greater than 4 because the analyzer will be run but it won't find any source files to process (example).

Therefore, it's important that this variable is in the documentation where users can find it.

Related issues

See above + internal slack thread.

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