[WIP] Revise documentation for Swift 5 stdlib additions (#21333)
Note: This cherry-pick drops the documentation revisions to the
Result type, which isn't included on this branch.
* Revise the Unicode scalar/Character properties
* Minor revisions to `compactMapValues` docs.
* Add documentation for AdditiveArithmetic, revise Numeric
* Apply minor style updates to count(where:).
* Revise string interpolation docs.
- Convert table of interpolation examples to a list of examples. Tables
aren't supported by Swift markup, so this wouldn't render properly in
Xcode or on the web.
- Add a description of what a user must implement in a custom
string interpolation type to get the behavior they want.
* Revise isMultiple(of:) docs.
- In particular, add emphasis to mathematical symbols and equations to
match how we document such things elsewhere.
- I'm using asterisks for single symbols, and underscores for equations
because it's easier to read in-source when you don't have to escape
multiplication within emphasis.
* Add some abstracts to the SIMD vector types.
- Adds a dictionary of spelled out numbers. Only numbers < 10
should be spelled out according to editorial.
- Adds abstracts to some of the basic members.
- Includes parameter descriptions for the xyzw properties and inits,
but not for the unlabeled initializers. Combined with the protocol
extension method abstracts, this should complete coverage of the concrete
types.
