Add documentation for emitted messages (#7936)

* Add doc for import-error
* Add doc for disallowed-name
* Add doc for else-if-used
* Add doc for import-self

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

doc/data/messages/d/disallowed-name/bad.py

@@ -0,0 +1,2 @@
+def foo():  # [disallowed-name]
+    pass

doc/data/messages/d/disallowed-name/good.py

@@ -1 +1,2 @@
-# This is a placeholder for correct code for this message.
+def something_meaningful():
+    pass

doc/data/messages/e/else-if-used/bad.py

@@ -0,0 +1,7 @@
+if input():
+    pass
+else:
+    if len(input()) >= 10:  # [else-if-used]
+        pass
+    else:
+        pass

doc/data/messages/e/else-if-used/good.py

@@ -1 +1,6 @@
-# This is a placeholder for correct code for this message.
+if input():
+    pass
+elif len(input()) >= 10:
+    pass
+else:
+    pass

doc/data/messages/e/else-if-used/pylintrc

@@ -0,0 +1,2 @@
+[MAIN]
+load-plugins=pylint.extensions.check_elif

doc/data/messages/i/import-error/bad.py

@@ -0,0 +1 @@
+from patlib import Path  # [import-error]

doc/data/messages/i/import-error/details.rst

@@ -1 +1,3 @@
-You can help us make the doc better `by contributing <https://github.com/PyCQA/pylint/issues/5953>`_ !
+This can happen if you're importing a package that is not installed in your environment, or if you made a typo.
+
+The solution is to install the package via pip/setup.py/wheel or fix the typo.

doc/data/messages/i/import-error/good.py

@@ -1 +1 @@
-# This is a placeholder for correct code for this message.
+from pathlib import Path

doc/data/messages/i/import-self/bad.py

@@ -0,0 +1,5 @@
+from bad import a_function  # [import-self]
+
+
+def a_function():
+    pass

doc/data/messages/i/import-self/good.py

@@ -1 +1,2 @@
-# This is a placeholder for correct code for this message.
+def a_function():
+    pass