Update documentation for the beCloseTo and beWithin matchers

[younata]

Solves #888.

Updates documentation for the beCloseTo matcher to correctly state that beCloseTo expects values to conform to FloatingPoint.
Adds documentation for the beWithin matcher.

Does not add documentation for toNever (nor toAlways), because my PR adding toAlways included that documentation.

• Does this have documentation?

[jessesquires]

@younata Is this the last thing needed for v10? 😄 🎉

Should we consider dropping old OSes and Xcodes for v10, just like Quick/Quick#1146?

Or do you think we should wait until v11?

[younata]

Is this the last thing needed for v10?

Other than writing the release notes, yes. 🎉 indeed!

Should we consider dropping old OSes and Xcodes for v10, just like Quick/Quick#1146?
Or do you think we should wait until v11?

Let's wait until v11 to drop old OSes and Xcodes. That way we can ship Nimble 10 with Quick 5.1 (or a 5.1.1 release, if you want). I filed #984 to track the work of dropping support for older Xcode/OSes.

[jessesquires]

That way we can ship Nimble 10 with Quick 5.1

Ah, perfect! 👌🏼