docs(lib/mocha.js): Update API documentation (mochajs#3515)

### Description of the Change

Replaced all method headers with more complete documentation, better following recommended styles. Eliminated `@api` tags, as `@access` _should_ imply same. Used `@chainable` to denote class methods returning `this` instance.

Renamed a handful of variables, but **no logic changes**!

### Alternate Designs

N/A

### Why should this be in core?

N/A

### Benefits

API documentation will be more useful if more complete.

### Possible Drawbacks

N/A

### Applicable issues

semver-patch