Added tags specialization

since the introduction of static analysis tools in php we see that
those tools (phpstan, psalm) both support prefixed tags in their
parsing. By explicit mention the already supported syntax for those
tags, we make sure to include those tool specs in the standard.

Author: jaapio

proposed/phpdoc.md

@@ -12,7 +12,8 @@ PSR-5: PHPDoc
   - [5.2. Description](#52-description)
     - [5.2.1 Inline tags](#521-inline-tags)
   - [5.3. Tags](#53-tags)
-    - [5.3.1. Tag Name](#531-tag-name)
+    - [5.3.1. Tag Specialization](#531-tag-specialization) 
+    - [5.3.2. Tag Name](#532-tag-name)
   - [5.4. Examples](#54-examples)
 - [Appendix A. Types](#appendix-a-types)
   - [ABNF](#abnf)
@@ -198,9 +199,9 @@ The PHPDoc format has the following [ABNF][RFC5234] definition:
                                                      ; with inline tags inside
     tags               = *(tag 1*eol)
     inline-tag         = "{" tag "}"
-    tag                = "@" tag-name [":" tag-specialization] [tag-details]
+    tag                = "@"[tag-specialization "-"] tag-name [tag-details]
     tag-name           = (ALPHA / "\") *(ALPHA / DIGIT / "\" / "-" / "_")
-    tag-specialization = 1*(ALPHA / DIGIT / "-")
+    tag-specialization = 1*(ALPHA / DIGIT)
     tag-details        = (1*SP tag-description)
     tag-description    = (CHAR / inline-tag) *(CHAR / inline-tag / eol)
     tag-argument       = *SP 1*CHAR [","] *SP
@@ -297,7 +298,21 @@ tags are semantically identical:
 
 This definition does NOT apply to _Annotation_ tags, which are not in scope.
 
-#### 5.3.1. Tag Name
+#### 5.3.1 Tag Specialization
+
+Tag specialization give a scope to the tag. It is RECOMMENDED to follow the
+list of tags in the tag [Tag Catalog PSR][TAG_PSR]. But the metadata of specialized
+tags MAY differ from the list.
+
+```php
+  /**
+   * @vendor-tag This is a description
+   */
+```
+
+Parsers MAY ignore specialized tags when they are in a supported format.
+
+#### 5.3.2. Tag Name
 
 Tag names indicate what type of information is represented by this tag.