docs: Document doctest +NUMBER limitation with strings

Also added an "xfail" testcase for the same.

doc/en/doctest.rst

@@ -152,7 +152,9 @@ pytest also introduces new options:
           0.23300000000000001
 
   ``NUMBER`` also supports lists of floating-point numbers -- in fact, it
-  supports floating-point numbers appearing anywhere in the output.
+  matches floating-point numbers appearing anywhere in the output, even inside
+  a string! This means that it may not be appropriate to enable globally in
+  ``doctest_optionflags`` in your configuration file.
 
 
 Continue on failure

testing/test_doctest.py

@@ -958,6 +958,9 @@ class TestLiterals:
             # Only the actual output is rounded up, not the expected output:
             ("3.0", "2.98"),
             ("1e3", "999"),
+            # The current implementation doesn't understand that numbers inside
+            # strings shouldn't be treated as numbers:
+            pytest.param("'3.1416'", "'3.14'", marks=pytest.mark.xfail),
         ],
     )
     def test_number_non_matches(self, testdir, expression, output):