Provide more hints about using the documentation

[sogaiu]

This PR contains changes that attempt to explain more about using Janet's documentation.

It includes:

• An addition explaining the use of the doc macro with strings to search for binding names as well as a sample invocation.
• Spelling out that each binding on the Core API page has certain info listed (e.g. a link to source, etc.).
• A note about how the docstrings assume that the reader has gotten some degree of familiarity of Janet. Additionally, some specific hints about what one might use to help interpret docstrings were added.
• Some shortening of existing text.
• Whitespace and reflow changes.

[pepe]

LGTM