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`.

main/main.h

@@ -76,7 +76,7 @@ public:
 	static void cleanup(bool p_force = false);
 };
 
-// Test main override is for the testing behaviour.
+// Test main override is for the testing behavior.
 #define TEST_MAIN_OVERRIDE                                         \
 	bool run_test = false;                                         \
 	int return_code = Main::test_entrypoint(argc, argv, run_test); \