Match more docstrings

[hristost]

Previously, certain keywords in comments would be left unmatched if the docstring was written as multiple single-line comments or if the comments began with an asterisk. For example:

/**
 * Function description
 * - Parameter a: parameter description
 * - Returns: return value description
 */

or

/// Function description
/// - Parameter a: parameter description
/// - Returns: return value description

This pull request fixes that, and furthermore introduces two new syntax groups: swiftDocStringParam, which matches parameter names in docstrings, and swiftDocStringInlineCode which matches text surrounded by back-ticks in comments. Those two groups are highlighted with SpecialComment by default, but by overriding them users should be able to get closer to the way Xcode displays comments.

Below is a screenshot with the improved highlighting:

Note how - Returns and - Parameter are highlighted in bold, and the parameter name and the zero in the last line are highlighted in a lighter colour.