Draft specification content for strong span

Write up the initial specification document content for the strong span. This content will also act as an example of the writing guidelines proposed in #34 and should be used as a working reference along with !17 (merged) (the paragraph block written up using the guidelines).

This issue requires:

• A brief non-normative description of the element
• Source examples included from the TCK
• Context section with content
• Content model section with content
• Attributes and metadata section with content
• ASG and DOM placeholder section

The issue requirements might change depending on feedback provided in issue #34.