Version 0.6.x Documentation Improvements

[hiltontj]

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

• No explanation in the Recursive Descent section
  ([#41] Add a line briefly describing descendant operator in docs #53)
• The second example in Filter Expressions seems like a poor use of the $ token in a filter expression ([#41] Add a better example to the Filter section in main docs #54)
• The slice selector section could use a separate example for reverse indices. (improve slice selector examples in docs #55)
• 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 The focus on existence and comparison is perhaps a bit outdated, and it may be useful to mention functions earlier on. ([#41] Documentation Improvements #56)
• Improvements to the functions module documentation #57
• The note about more lenient accessors could be moved to the end of the section. ([#41] Documentation Improvements #56)
• The description in serde_json_path crate Cargo.toml should use the word "query" instead of "parse" and omit the trailing "queries" (Release 0.6.2 #52)
• Update documentation to cover JSONPath compliance #58
• Code examples for methods in NodeList that do not have them, e.g., first, last, get ([#41] Documentation Improvements #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.