[v3] update parallel mode using docs

[Benjamin Kosnik]

This clarifies the use of "a la carte" parallel algorithms, ie just
making std::sort parallel. This is in response to some user feedback.

In addition, I noticed that the stubs that I added for exception
handling documentation made the manual no longer validate. Fixed, and
regenerated the html docs at the same time.

tested x86/linux
validated using "make doc-xml-validate"

-benjamin
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