Replace "inverse sqrt" with "reciprocal" sqrt

[Pennycook]

The "inverse" of sqrt is x*x. Referring to "reciprocal" sqrt is more accurate.

Closes #655.

[TApplencourt]

Agree, and it's even in the name of the function rsqrt :)

[tomdeakin]

This is much clearer wording for 1/sqrt(x).
Probably just worth a quick check by WG that we do mean this and not inverse.

[gmlueck]

I think this is OK because the name of the function is "rsqrt" (not "isqrt"). However:

• OpenCL names this function "inverse square root".
• I think the term "inverse square root" is commonly used. For example, see this Wikipedia article.
• According to Google search "inverse square root" is about 5 times more commonly used than "reciprocal square root".

I checked the wording in IEEE 754 to see what they call this function, but they do not use any English description. They just name the function "rsqrt".

[tomdeakin]

WG approved to merge.