PM / Freezer / Docs: Document the beauty of freeze/thaw semantics

order to clear the PF_FROZEN flag for each frozen task.  Then, the tasks that

have been frozen leave __refrigerator() and continue running.

Rationale behind the functions dealing with freezing and thawing of tasks:

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freeze_processes():

  - freezes only userspace tasks

freeze_kernel_threads():

  - freezes all tasks (including kernel threads) because we can't freeze

    kernel threads without freezing userspace tasks

thaw_kernel_threads():

  - thaws only kernel threads; this is particularly useful if we need to do

    anything special in between thawing of kernel threads and thawing of

    userspace tasks, or if we want to postpone the thawing of userspace tasks

thaw_processes():

  - thaws all tasks (including kernel threads) because we can't thaw userspace

    tasks without thawing kernel threads

III. Which kernel threads are freezable?

Kernel threads are not freezable by default.  However, a kernel thread may clear