The problem-matchers documentation is missing a lot of information

The file https://github.com/actions/toolkit/blob/master/docs/problem-matchers.md is missing a lot of relevant information. Here are some questions it does not answer:

1. The docs say: "column: a group number containing the column information". What is the valid format for "column information"? Is it used anywhere? Is it displayed anywhere?
2. The docs say "code: a group number containing the error code". Is this the exit code? Any sort of number associated with the code? Is it used anywhere?
3. The docs says "at least one pattern must set the message". What happens if multiple patterns set the message? Are they concatenated? Is all but the last one dropped?
4. The docs say "file: a group number containing the file name". Does the file name need to be absolute? Relative to the root of the repo? Relative to "fromPath" (which maybe defaults to the root of the repo)? Either absolute or relative? And what happens if the file does not exist? Or exists but not in a file touched by the PR? Or exists but only in a submodule? Or contains special characters such as "? (Empirically, this last one (or possibly non-existent file) seems to result in the file .github being chosen instead.)

Start with docs/problem-matchers.md and the problem-matchers section of docs/commands.md. Investigate each listed question against the toolkit's existing behavior, then update problem-matchers.md with the missing semantics, links, and documented behavior for matcher fields, patterns, paths, and invalid files.