Version 0.6.x Documentation Improvements

[hiltontj]

The purpose of this issue is to track some necessary improvements/fixes in the crate documentation.

• [x] No explanation in the Recursive Descent section (#53)
• [x] The second example in Filter Expressions seems like a poor use of the $ token in a filter expression (#54)
• [x] The slice selector section could use a separate example for reverse indices. (#55)
• [x] The filter selectors section should be generally re-worded. The focus on existence and comparison is perhaps a bit outdated, and it may be useful to mention functions earlier on. (#56)
• [x] #57
• [x] The note about more lenient accessors could be moved to the end of the section. (#56)
• [x] The description in serde_json_path crate Cargo.toml should use the word "query" instead of "parse" and omit the trailing "queries" (#52)
• [x] #58
• [x] Code examples for methods in NodeList that do not have them, e.g., first, last, get (#56)

[hiltontj]

Two items in this issue have been converted to separate issues (#57 and #58). Otherwise, all documentation updates have been made, so this can be closed.