diff --git a/clang/docs/analyzer/checkers.rst b/clang/docs/analyzer/checkers.rst
--- a/clang/docs/analyzer/checkers.rst
+++ b/clang/docs/analyzer/checkers.rst
@@ -2333,6 +2333,43 @@
 alpha.unix
 ^^^^^^^^^^^
 
+.. _alpha-unix-StdCLibraryFunctionArgs:
+
+alpha.unix.StdCLibraryFunctionArgs (C)
+""""""""""""""""""""""""""""""""""""""
+Check for calls of standard library functions that violate predefined argument
+constraints. For example, it is stated in the C standard that for the ``int
+isalnum(int ch)`` function the behavior is undefined if the value of ``ch`` is
+not representable as unsigned char and is not equal to ``EOF``.
+
+.. code-block:: c
+
+  void test_alnum_concrete(int v) {
+    int ret = isalnum(256); // \
+    // warning: Function argument constraint is not satisfied
+    (void)ret;
+  }
+
+If the argument's value is unknown then the value is assumed to hold the proper value range.
+
+.. code-block:: c
+
+  #define EOF -1
+  void test_alnum_symbolic(int x) {
+    int ret = isalnum(x);
+    (void)ret;
+    clang_analyzer_eval(EOF <= x && x <= 255); // this reports TRUE
+  }
+
+If the user disables the checker then the argument violation warning is
+suppressed. However, the assumption about the argument is still modeled (otherwise we
+would be further analyzing an illformed program).
+
+The checker models functions (and emits diagnostics) from the C standard by
+default. The ``ModelPOSIX`` option enables the checker to model (and emit
+diagnostics) for functions that are defined in the POSIX standard. This option
+is disabled by default.
+
 .. _alpha-unix-BlockInCriticalSection:
 
 alpha.unix.BlockInCriticalSection (C)
diff --git a/clang/include/clang/StaticAnalyzer/Checkers/Checkers.td b/clang/include/clang/StaticAnalyzer/Checkers/Checkers.td
--- a/clang/include/clang/StaticAnalyzer/Checkers/Checkers.td
+++ b/clang/include/clang/StaticAnalyzer/Checkers/Checkers.td
@@ -552,7 +552,7 @@
            "or is EOF.">,
   Dependencies<[StdCLibraryFunctionsChecker]>,
   WeakDependencies<[CallAndMessageChecker, NonNullParamChecker, StreamChecker]>,
-  Documentation<NotDocumented>;
+  Documentation<HasAlphaDocumentation>;
 
 } // end "alpha.unix"