Mention documentation comments in GDScript basics

[Calinou]

• See tutorials/scripting/gdscript/gdscript_exports godot-docs-user-notes#143 (comment).

[tetrapod00]

This seems fine to merge as-is, but I have two possible suggestions to bring this more in line with the rest of the page.

• Add a code block with the most common examples.
• Shorten the paragraph and just link to the main page, as is done for Exports and History.

[tetrapod00]

Suggested change

	It is also possible to mark certain comments as
	:ref:`documentation comments <doc_gdscript_documentation_comments>` by using
	two hash symbols (``##``) instead of one (``#``). This only has an effect for
	comments placed immediately *above* a documentable item (such as a member
	variable), or at the top of a file. Dedicated formatting options are also
	available. See :ref:`doc_gdscript_documentation_comments` for details.
	Use two hash symbols (``##``) instead of one (``#``) to add a
	:ref:`documentation comment <doc_gdscript_documentation_comments>`, which will
	appear in the script documentation and in the inspector description of an
	exported variable. Documentation comments must be placed directly *above* a
	documentable item (such as a member variable), or at the top of a file.
	Dedicated formatting options are also available.
	See :ref:`doc_gdscript_documentation_comments` for details.
	::
	## This comment will appear in the script documentation.
	var value
	## This comment will appear in the inspector tooltip, and in the documentation.
	@export var exported_value

(And also move this whole section to before the tip about special keywords, so the text flows better.)

I also changed the phrasing a bit to be more active and succinct.

Linking to the same page twice in a paragraph does seem a bit odd, but it's fine.

[tetrapod00]

Suggested change

	It is also possible to mark certain comments as
	:ref:`documentation comments <doc_gdscript_documentation_comments>` by using
	two hash symbols (``##``) instead of one (``#``). This only has an effect for
	comments placed immediately *above* a documentable item (such as a member
	variable), or at the top of a file. Dedicated formatting options are also
	available. See :ref:`doc_gdscript_documentation_comments` for details.
	It is also possible to mark certain comments as documentation
	comments by using two hash symbols (``##``) instead of one (``#``).
	See :ref:`doc_gdscript_documentation_comments` for details.

Alternately, following some other examples on this page, like History or Exports, only include a very minimal link to the page with the actual description, and don't link to the same page twice in a paragraph.