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`.

tests/core/input/test_input_event_key.h

@@ -102,7 +102,7 @@ TEST_CASE("[InputEventKey] Key correctly converts itself to text") {
 	// as text. These cases are a bit weird, since None has no textual representation
 	// (find_keycode_name(Key::NONE) results in a nullptr). Thus, these tests look weird
 	// with only (Physical) or a lonely modifier with (Physical) but (as far as I
-	// understand the code, that is intended behaviour.
+	// understand the code, that is intended behavior.
 
 	// Key is None without a physical key.
 	none_key.set_keycode(Key::NONE);