Document GenericMemory and AtomicMemory (#54642)
Closes https://github.com/JuliaLang/julia/issues/53854. After talking
with @vtjnash, we are ready to commit to the `GenericMemory` interface.
Sorry @nsajko that this took me so long to get around to.
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Co-authored-by: Marek Kaluba <kalmar@mailbox.org>
Co-authored-by: Neven Sajko <s@purelymail.com>
