[analyzer][docs] Document alpha.security.cert.pos.34c limitations

[steakhal]

PS: the html docs compile and look great.

[martong]

LGTM!

clang/docs/analyzer/checkers.rst

int func(const char *var) {
     int retval = snprintf(env, sizeof(env),"TEST=%s", var);
     if (retval < 0 || (size_t)retval >= sizeof(env)) {
         /* Handle error */
     }
 
     return putenv(env); // putenv function should not be called with auto variables
   }
 
+Limitations:
+
+   - Technically, one can pass automatic variables to ``putenv``,
+     but one needs to ensure that the given environment key stays
+     alive until it's removed or overwritten.
+     Since the analyzer cannot keep track of which envvars get overwritten
+     and when, it needs to be slightly more aggressive and warn for such
+     cases too, leading in some cases to false-positive reports like this:
+
+     .. code-block:: c
+
+        void baz() {
+          char env[] = "NAME=value";
+          putenv(env); // false-positive warning: putenv function should not be called...
+          // More code...
+          putenv((char *)"NAME=anothervalue");
+          // This putenv call overwrites the previous entry, thus that can no longer dangle.
+        } // 'env' array becomes dead only here.
+
 alpha.security.cert.env
 ^^^^^^^^^^^^^^^^^^^^^^^
 
 SEI CERT checkers of `Environment C coding rules <https://wiki.sei.cmu.edu/confluence/x/JdcxBQ>`_.
 
 .. _alpha-security-cert-env-InvalidPtr:
 
 alpha.security.cert.env.InvalidPtr