Help: Clarify {PASS,FAIL}_REGULAR_EXPRESSION semantics w.r.t. exit code

Also cross-reference them with each other and `SKIP_REGULAR_EXPRESSION`.

Help/prop_test/FAIL_REGULAR_EXPRESSION.rst

@@ -1,7 +1,8 @@
 FAIL_REGULAR_EXPRESSION
 -----------------------
 
-If the output matches this regular expression the test will fail.
+If the output matches this regular expression the test will fail,
+regardless of the process exit code.
 
 If set, if the output matches one of specified regular expressions,
 the test will fail.  Example:
@@ -13,3 +14,6 @@ the test will fail.  Example:
   )
 
 ``FAIL_REGULAR_EXPRESSION`` expects a list of regular expressions.
+
+See also the :prop_test:`PASS_REGULAR_EXPRESSION` and
+:prop_test:`SKIP_REGULAR_EXPRESSION` test properties.

Help/prop_test/PASS_REGULAR_EXPRESSION.rst

@@ -2,6 +2,7 @@ PASS_REGULAR_EXPRESSION
 -----------------------
 
 The output must match this regular expression for the test to pass.
+The process exit code is ignored.
 
 If set, the test output will be checked against the specified regular
 expressions and at least one of the regular expressions has to match,
@@ -14,3 +15,6 @@ otherwise the test will fail.  Example:
   )
 
 ``PASS_REGULAR_EXPRESSION`` expects a list of regular expressions.
+
+See also the :prop_test:`FAIL_REGULAR_EXPRESSION` and
+:prop_test:`SKIP_REGULAR_EXPRESSION` test properties.

Help/prop_test/SKIP_REGULAR_EXPRESSION.rst

@@ -16,4 +16,6 @@ the test will be marked as skipped.  Example:
 
 ``SKIP_REGULAR_EXPRESSION`` expects a list of regular expressions.
 
-See also the :prop_test:`SKIP_RETURN_CODE` property.
+See also the :prop_test:`SKIP_RETURN_CODE`,
+:prop_test:`PASS_REGULAR_EXPRESSION`, and :prop_test:`FAIL_REGULAR_EXPRESSION`
+test properties.