Improve doc of URI.encode

[c4710n]

This PR aims to improve doc of URI.encode:

• Provide a clearer explanation for the predicate argument, including its execution logic and the built-in functions that can be used.
• List common use cases for the built-in functions. For example, URI.char_unescaped?/1 for the whole URL, URI.char_unreserved?/1 for URI components.
• Correct the line - "See encode_www_form/1 if you are interested in escaping reserved
  characters too.", which is not very precise:
  • if we want to escape reserved characters, we should use URI.char_unreserved?/1, not URI.encode_www_form/1.
  • We only use URI.encode_www_form/1 when we want to encode a string as "x-www-form-urlencoded".
• Remove the explanation of truthy and falsy values, as we should assume developers are already familiar with them. Otherwise, we would have to explain them every time they appear.

Feel free to comment, and I'd like to make changes.