Clarify SNAME usage

* Explain where it should be used, with examples.
* Clarify that it should _not_ be used everywhere, only where needed.
* Supersedes #57720

This PR is the result of the discussion that happened in a contractor meeting, and it attempts to clarify the intended use for this macro for other contributors.
As a personal note, It is my view that other approaches to using SNAME (like having a global or per class table of string names) are mere overengineering without any real benefit (performance remains the same, and usage of stringnames becomes more cumbersome. Additionally, there was not any significant amount of errors in name mismatching as a result of using strings since Godot was open sourced).

core/string/string_name.h

@@ -181,6 +181,18 @@ bool operator!=(const char *p_name, const StringName &p_string_name);
 
 StringName _scs_create(const char *p_chr, bool p_static = false);
 
+/*
+ * The SNAME macro is used to speed up StringName creation, as it allows caching it after the first usage in a very efficient way.
+ * It should NOT be used everywhere, but instead in places where high performance is required and the creation of a StringName
+ * can be costly. Places where it should be used are:
+ * - Control::get_theme_*(<name> and Window::get_theme_*(<name> functions.
+ * - emit_signal(<name>,..) function
+ * - call_deferred(<name>,..) function
+ * - Comparisons to a StringName in overriden _set and _get methods.
+ *
+ * Use in places that can be called hundreds of times per frame (or more) is recommended, but this situation is very rare. If in doubt, do not use.
+ */
+
 #define SNAME(m_arg) ([]() -> const StringName & { static StringName sname = _scs_create(m_arg, true); return sname; })()
 
 #endif // STRING_NAME_H