Clarify PVP/SemVer comparison

Four-component versioning (A.B.C.D) with `D` being patch-level change
(lower risk than minor-level change) is ubiquitous, but not mandated
by the PVP.

Author: ffaf1

pvp-faq.md

@@ -29,9 +29,13 @@ The most apparent somewhat superficial difference between SemVer and the
 PVP is which components of the version number are interpreted as
 major, minor, and patch-level numerals:
 
- - PVP: *MAJOR.MAJOR.MINOR.PATCH*
+ - PVP: *MAJOR.MAJOR.MINOR.PATCH* [^1]
  - SemVer: *MAJOR.MINOR.PATCH*
 
+[^1]: In PVP, components beyond the third are optional, to be used in any
+      way the package maintainer sees fit. In practice, four-component
+      versioning (`A.B.C.D`) is standard.
+
 While having a 2-part major component may confuse people used to
 SemVer, it provides an additional dimension for communicating API
 changes: