Clarify some docs

Author: steakhal

clang/docs/analyzer/checkers.rst

@@ -1757,19 +1757,25 @@ unix.Chroot (C)
 Check improper use of chroot described by SEI Cert C recommendation `POS05-C.
 Limit access to files by creating a jail
 <https://wiki.sei.cmu.edu/confluence/display/c/POS05-C.+Limit+access+to+files+by+creating+a+jail>`_.
-The checker finds usage patterns where chdir() is not called immediately
-after a call to chroot().
+The checker finds usage patterns where ``chdir("/")`` is not called immediately
+after a call to ``chroot(path)``.
 
 .. code-block:: c
 
  void f();
 
- void test() {
+ void test_bad() {
    chroot("/usr/local");
    f(); // warn: no call of chdir("/") immediately after chroot
  }
 
- void test() {
+  void test_bad_path() {
+    chroot("/usr/local");
+    chdir("/usr"); // warn: no call of chdir("/") immediately after chroot
+    f();
+  }
+
+ void test_good() {
    chroot("/usr/local");
    chdir("/"); // no warning
    f();

clang/test/Analysis/chroot.c

@@ -58,3 +58,11 @@ void f7(void) {
       // expected-note@-2    {{No call of chdir("/") immediately after chroot}}
   }
 }
+
+void f8() {
+  chroot("/usr/local"); // expected-note {{chroot called here}}
+  chdir("/usr"); // This chdir was ineffective because it's not exactly `chdir("/")`.
+  foo();
+  // expected-warning@-1 {{No call of chdir("/") immediately after chroot}}
+  // expected-note@-2    {{No call of chdir("/") immediately after chroot}}
+}