manual: Use more precise wording for memory protection keys

manual: Use more precise wording for memory protection keys

Update the name of the argument in several pkey_*() functions that refers
to access restrictions rather than access rights: change access "rights"
to access "restrictions".

Specify that the result of the pkey_get() should be checked using bitwise
operations rather than plain equals comparison.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>