[NFC][analyzer] Document optin.portability.UnixAPI (llvm#150005)

This commit provides a brief documentation for the checker
optin.portability.UnixAPI.

Unfortunately the name of this checker is meaninglessly vague and its
functionality is very closely related to unix.Malloc, so it should be
eventually "rebranded" to a more user-friendly presentation.

Author: NagyDonat

clang/docs/analyzer/checkers.rst

@@ -1103,7 +1103,16 @@ To override this threshold to e.g. 4 bytes, use the
 
 optin.portability.UnixAPI
 """""""""""""""""""""""""
-Finds implementation-defined behavior in UNIX/Posix functions.
+Reports situations where 0 is passed as the "size" argument of various
+allocation functions ( ``calloc``, ``malloc``, ``realloc``, ``reallocf``,
+``alloca``, ``__builtin_alloca``, ``__builtin_alloca_with_align``, ``valloc``).
+
+Note that similar functionality is also supported by :ref:`unix-Malloc` which
+reports code that *uses* memory allocated with size zero.
+
+(The name of this checker is motivated by the fact that it was originally
+introduced with the vague goal that it "Finds implementation-defined behavior
+in UNIX/Posix functions.")
 
 
 optin.taint

clang/include/clang/StaticAnalyzer/Checkers/Checkers.td

@@ -1656,8 +1656,8 @@ def CloneChecker : Checker<"CloneChecker">,
 let ParentPackage = PortabilityOptIn in {
 
 def UnixAPIPortabilityChecker : Checker<"UnixAPI">,
-  HelpText<"Finds implementation-defined behavior in UNIX/Posix functions">,
-  Documentation<NotDocumented>;
+  HelpText<"Finds dynamic memory allocation with size zero">,
+  Documentation<HasDocumentation>;
 
 } // end optin.portability