Naming and Describing Section: Revisions to address feedback in issue…

… 1050 (pull #1062)

* Fallback naming guidance section: reference cardinal rule 4 to avoid it.
* Naming Rule 3: Remove mention of alt attribute: Per feedback from @LJWatson in issue #1050, revises rule 3 to remove mention of alt.
* Adds sentence refering readers to the naming techniques for specific advantages of using HTML features instead of ARIA.
* Fix formatting and bugs in code snippets reported in #1060.
* Intro: most fundamental and important -> most important
* Minor editorial change to section: How Are Name and Description Strings Derived?
* Intro paragraph: additional editorial simplification based on feedback from @LJWatson in issue #1050.
* Clarify language describing when captions become descriptions: to resolve feedback from @jessebeach in issue #1050. Modified language in sections 5.3.2.6 Naming Tables and Figures with Captions  and  section 5.4.1.3 Describing Tables and Figures with Captions.
* Provide richer description of screen reader behavior in section 5.1 per feedback from @jongund in issue #1050.
* Remove aria-details guidance, which will be addressed in the future by issue #70.