Document the move of the third-party extensions to doc-extensions

docs/faq.md

@@ -1,6 +1,10 @@
 # Frequently Asked Questions
 
 ## I'm about to document a new PHP extension. How should I start?
+Extensions bundled with PHP are documented in doc-en. Third-party extensions
+(typically distributed via PECL or PIE) are documented in
+[doc-extensions](https://github.com/php/doc-extensions).
+
 Change your working directory to `phpdoc/doc-base/scripts/docgen/` and execute:
 ```sh
 php docgen.php -e simplexml -o outdir

docs/overview.md

@@ -8,6 +8,8 @@ All parts of the process live in the following GitHub repositories:
    these are the repositories where all the information on documentation pages live
    and this is where changes to the wording and/or structure
    of the documentation's content can be made
+- [**doc-extensions**](#doc-extensions): the documentation of the third-party extensions,
+   in English only
 - [**doc-base**](#doc-base): validates and pre-processes the language files
 - [**PhD**](#phd): renders pages into .php files which include information on the files
    and function calls to include the documentation page headers and footers
@@ -24,6 +26,16 @@ of the [php.net's documentation pages](https://www.php.net/docs.php) are made.
 Please refer to the [contribution guidlines](contributing.md) on how to
 contribute to the documentation.
 
+## doc-extensions
+
+[doc-extensions](https://github.com/php/doc-extensions) contains the documentation of the
+third-party extensions, which are maintained outside the [php-src](https://github.com/php/php-src)
+repository (typically distributed via PECL or PIE). It is rendered as its own manual
+at [php.net/manual/extensions/](https://www.php.net/manual/extensions/).
+
+Only the English documentation is maintained there: the translations of these extensions
+were not carried over from the language repositories.
+
 ## doc-base
 
 [doc-base](https://github.com/php/doc-base) validates and combines the documentations' XML files.

docs/structure.md

@@ -30,7 +30,7 @@ A procedural extension (like calendar) also has:
 - *reference.xml* - container for the functions, rarely contains any info
 - *functions/* - folder with one XML file per function that the extension declares
 
-And OO extensions (such as imagick) contain:
+And OO extensions (such as dom) contain:
 - *classname.xml* - container for the methods defined by the class, contains also basic info about it
 - *classname/* - folder with one XML file per method that the class declares
 

docs/translating.md

@@ -6,6 +6,9 @@ You will also have to follow other steps from the [editing manual sources](editi
 Translating documentation into other languages might look like a complicated
 process, but in fact, it's rather simple.
 
+The documentation of third-party extensions lives in
+[doc-extensions](https://github.com/php/doc-extensions) and is not translated.
+
 Because any files that are not yet translated are automatically included
 from the English version of the manual when the HTML and other versions
 of the translated manual are built, only files that are translated should