Document "auto" value in the manual's multi-threading section

[hammer]

#35108 and #38952 together add the ability to use auto to set the number of threads for Julia at startup with a command-line flag or environment variable.

I think the Starting Julia with multiple threads subsection of the Multi-threading section of the Julia manual could be improved by a mention of the convenient auto value.

I am no expert so I defer to y'all on whether and how to add this information. Thanks for the consideration!

[felixcremer]

I think, that this is closed in #42501