Documentation: review of the @see tags

[jrfnl]

Pull Request Type

• I have checked there is no other PR open for the same change.

This is a:

• Bug fix
• New feature
• Code quality improvement

Context

Improving the generated documentation for the website.

Detailed Description

Docs: use @link tags for external references

Docs: use parentheses when referencing a function in a @see tag

Docs: use fully qualified names in @see tags [1]

... for Requests native classes to prevent confusion.

Docs: use fully qualified names in @see tags [2]

... for external (PHP native) classes.

Previously, APIGen would add an incorrect link for this reference, which had to be removed for each website update.
With this change, that manual update can now hopefully be prevented.

Quality assurance

• This change does NOT contain a breaking change (fix or feature that would cause existing functionality to change).
• I have commented my code, particularly in hard-to-understand areas.
• I have added unit tests to accompany this PR.
• The (new/existing) tests cover this PR 100%.
• I have (manually) tested this code to the best of my abilities.
• My code follows the style guidelines of this project.

Documentation

For new features:

• I have added a code example showing how to use this feature to the examples directory.
• I have added documentation about this feature to the docs directory.
  If the documentation is in a new markdown file, I have added a link to this new file to the Docs folder README.md file.