docs: fix validation include line numbers (#418)

* docs: fix validation include line numbers

* use start/end markers instead of hardcoding lineno

* Add missing sentinel + explanatory comment.

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

doc/validation.rst

@@ -64,4 +64,5 @@ a more detailed message. For example::
 The full mapping of validation checks is given below.
 
 .. literalinclude:: ../numpydoc/validate.py
-   :lines: 36-90
+   :start-after: start-err-msg
+   :end-before: end-err-msg

numpydoc/validate.py

@@ -34,6 +34,9 @@
     "References",
     "Examples",
 ]
+# NOTE: The following comment is a sentinel for embedding in the docs - do not
+# modify/remove
+# start-err-msg
 ERROR_MSGS = {
     "GL01": "Docstring text (summary) should start in the line immediately "
     "after the opening quotes (not in the same line, or leaving a "
@@ -91,6 +94,9 @@
     "SA04": 'Missing description for See Also "{reference_name}" reference',
     "EX01": "No examples section found",
 }
+# end-err-msg
+# NOTE: The above comment is a sentinel for embedding in the docs - do not
+# modify/remove
 
 # Ignore these when evaluating end-of-line-"." checks
 IGNORE_STARTS = (" ", "* ", "- ")