doc: Add clarification to common execution contexts docs
doc: Add clarification to common execution contexts docs

A couple paragraphs contain ambiguous verbs that are unclear.
(For example: Line 23. Does the kernel perform a busy-wait,
or does it merely permit a task or fiber to perform a busy-wait?)
Based on original, I guess the latter and attempted to correct this
in the docs.

Also fixed spelling error.

Change-Id: I0699835a9681c1a4873575fbf9a69bc6d854bee4
Signed-off-by: L.S. Cook <2c8ffdbb27eb9faeeac367751f77804563aa8470@intel.com>

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