Update lock hierarchy devdocs (#58884)

Brings the devdocs up to date for the lock hierarchy we have now,
documenting the deadlock avoidance algorithm we use for method locks.
The information about which locks to take to update global data has been
removed because it was out of date, and some basic comments about
locking has been added to `julia.h`.

Also removes an unused mutex, `extern_c_lock`, which I found while
looking for locks.

TODO:
- A few fields that have questionable locking (`usings_backedges`,
`jl_binding_t.backedges`) will be fixed later.
- Document how the atomic fields are updated/read.

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Co-authored-by: Jameson Nash <vtjnash@gmail.com>

Author: xal-0