newdoc: add comment about the ext:*load-hooks*

src/doc/new-doc/standards/index.txi

@@ -114,6 +114,34 @@
 
 @node System construction
 @section System construction
+@c ext:*load-hooks*, si::*load-search-list* variable:
+
+@c EXT:*LOAD-HOOKS* is an assoc array of form ((TYPE . LOAD-FUNCTION)),
+@c where TYPE is either a string (i.e "lisp", "fasb" etc.), wildcard
+@c :WILD (matching any extension) and NIL for no
+@c extension. LOAD-FUNCTION is a symbol of a function used to load the
+@c file of the TYPE type.
+
+@c If the argument SOURCE of LOAD is a stream, it is read as an ordinary
+@c lisp source code, otherwise it should a pathname (or a string which
+@c may be coerced to it).
+
+@c If pathname doesn't have a directory, host nor device components,
+@c then file is looked in the `:SEARCH-LIST` directories (defaulting to
+@c si::*load-search-list*) and if found – loaded with LOAD (with
+@c pathname with a directory merged from the search-list).
+
+@c Otherwise (if a pathname does have a directory or the file can't be
+@c found in the SEARCH-LIST) and the file type is neither NIL or :WILD,
+@c then the assoc value of the TYPE is looked up in EXT:*LOAD-HOOKS* and
+@c funcalled on the file (if the TYPE doesn't exist, we load a file as a source code).
+
+@c If file type is NIL or :WILD, then we try to "guess" it's extension
+@c trying extensions from the EXT:*LOAD-HOOKS* in order in which they
+@c appear on the list. By default, first entry is (NIL
+@c . SI:LOAD-SOURCE), so if there is a file without extension in the
+@c directory, it will be treated as a source code. Otherwise we'll try
+@c known extensions.
 
 @node Environment
 @section Environment