Expand documentation on matching elements.

[wearhere]

Covering:

• where do accessibility properties come from?
• which accessibility properties are best to use?
• how can I match elements whose labels I don't know till runtime? (i.e. to assert they exist)

Some of this information is already covered in the documentation of SLElement's constructors, but could still be pulled out into an overview on the wiki. See this comment.

[MaxGabriel]

Working on this.

Some other things that probably are worth calling out:

• Matching children views of particular view, when Ax parent #190 is merged
• Unusual matching cases, like:
  • Matching UISegmentedControls or UISteppers (answer: match their individual buttons, with an explanation of how to set accessibility properties for those controls)
  • Matching elements in a webview (answer: use alt text)
• Remote View Controllers (answer: suspect this isn't possible; not sure)
• Accessibility Containers

[wearhere]

👍!