Style: Misc docs and comment style and language fixes

- Removed empty paragraphs in XML.
- Consistently use bold style for "Example:", on a new line.
- Fix usage of `[code]` when hyperlinks could be used (`[member]`, `[constant]`).
- Fix invalid usage of backticks for inline code in BBCode.
- Fix some American/British English spelling inconsistencies.
- Other minor fixes spotted along the way, including typo fixes with codespell.
- Don't specify `@GlobalScope` for `enum` and `constant`.

doc/classes/XRInterfaceExtension.xml

@@ -138,7 +138,7 @@
 		<method name="_is_initialized" qualifiers="virtual const">
 			<return type="bool" />
 			<description>
-				Returns [code]true[/code] if this interface has been initialised.
+				Returns [code]true[/code] if this interface has been initialized.
 			</description>
 		</method>
 		<method name="_notification" qualifiers="virtual">