Introduce recommendations for external specification modules (#605)

* Introduce recommendations for external specification modules

Bug: #506, #587

* Update index.bs

Co-authored-by: Alex Rudenko <[email protected]>

* remove external spec dfn

* Update index.bs

Co-authored-by: jgraham <[email protected]>

* Update index.bs

* remove paragraph

* linkify command names and event names

* Update index.bs

Co-authored-by: Alex Rudenko <[email protected]>

* define -> extend

---------

Co-authored-by: Alex Rudenko <[email protected]>
Co-authored-by: jgraham <[email protected]>

index.bs

@@ -474,8 +474,15 @@ An implementation may define <dfn export>extension modules</dfn>. These must hav
 [=module name=] that contains a single colon "<code>:</code>" character. The
 part before the colon is the prefix; this is typically the same for all
 [=extension modules=] specific to a given implementation and should be unique for a
-given implementation. Such modules extend the [=local end definition=] and [=remote end definition=]
-providing additional groups as choices for the defined [=commands=] and [=events=].
+given implementation.
+
+Other specifications may define their own WebDriver-BiDi modules that extend the protocol.
+Such modules must not have a name which contains a colon (<code>:</code>) character,
+nor must they define [=command names=], [=event names=], or property names that contain that character.
+
+Authors of external specifications are encouraged to to add new modules rather than extending
+existing ones. Where it is desired to extend an existing module, it is preferred to integrate the
+extension directly into the specification containing the original module definition.
 
 ## Commands ## {#commands}