Move options to common group, and add help text for options

[nickodell]

This PR aims to improve two things.

First, currently some options are in a dedicated 'run-parallel' section, but other options are in the 'Custom options' section. This seems confusing.

run-parallel:
  --parallel-threads=PARALLEL_THREADS
                        Set the number of threads used to execute each test concurrently.
  --iterations=ITERATIONS
                        Set the number of iterations that each thread will run.

Custom options:
  --skip-thread-unsafe=SKIP_THREAD_UNSAFE
                        Whether to skip running thread-unsafe tests
  --mark-warnings-as-unsafe
  --mark-ctypes-as-unsafe
  --mark-hypothesis-as-unsafe
  --ignore-gil-enabled
  --lsof                Run FD checks if lsof is available
  --runpytest={inprocess,subprocess}
                        Run pytest sub runs in tests using an 'inprocess' or 'subprocess' (python -m main) method

This can be improved by moving all of the options to the same group.

Second, some options could use help text to describe what they do. I also added documentation about the default behavior, in cases where it wasn't obvious.

• For --skip-thread-unsafe: I tried to clarify that the the alternative to skipping thread unsafe tests is running them in a single thread, not ignoring the thread safety problem.
• For --mark-warnings-as-unsafe: a user might think that if they don't pass this option, then warnings will not be marked as unsafe. However, this is not true. Also, I tried to clarify that the thing it reacts to is not warnings, per se, but warnings capture.
• For --mark-ctypes-as-unsafe: I added a similar note, but was worried it might imply that we check the C code that ctypes is calling as being thread-safe, which we don't, so I added the text "(but not the underlying C code)"
• For --ignore-gil-enabled: I added a note about what exactly this option protects against. I tried to emphasize that users who are running GIL enabled builds don't need to enable this option.

Here is the rendered revised help:

run-parallel:
  --parallel-threads=PARALLEL_THREADS
                        Set the number of threads used to execute each test
                        concurrently. (default: 1)
  --iterations=ITERATIONS
                        Set the number of iterations that each thread will run.
                        (default: 1)
  --skip-thread-unsafe=SKIP_THREAD_UNSAFE
                        Whether to skip running thread-unsafe tests. If not
                        provided, thread-unsafe tests will still run, but only in
                        one thread.
  --mark-warnings-as-unsafe
                        Mark warnings capture, such as pytest.warns(), as thread-
                        unsafe. If not provided, the thread safety of warnings
                        capture will be determined automatically.
  --mark-ctypes-as-unsafe
                        Mark all uses of ctypes as thread-unsafe. If not
                        provided, the thread safety of ctypes (but not the
                        underlying C code) will be determined automatically.
  --mark-hypothesis-as-unsafe
                        Mark hypothesis as thread-unsafe. If not provided, the
                        thread safety of hypothesis will be determined
                        automatically.
  --ignore-gil-enabled  Ignore the GIL becoming enabled in the middle of a test.
                        By default, if the GIL is re-enabled at runtime, pytest
                        will exit with a non-zero exit code. This option has no
                        effect for non-free-threaded builds.

[rgommers]

This will be quite helpful, thanks @nickodell!

[ngoldbaum]

Thanks @nickodell, I really appreciate all the nice cleanups and fixes you've been sending in!