Manual update: custom exceptions (#12847) [backport]

Said that you can have custom exceptions and showed how to create and raise them.

(cherry picked from commit 73dd348ddd)

doc/manual.rst

@@ -4126,6 +4126,23 @@ error message from ``e``, and for such situations it is enough to use
   except:
     echo getCurrentExceptionMsg()
 
+Custom exceptions
+-----------------
+
+Is it possible to create custom exceptions. These make it easy to distinguish between exceptions raised by nim and those from your own code.
+
+A custom exception is a custom type:
+
+.. code-block:: nim
+  type
+    LoadError* = object of Exception
+  
+Ending the custom exception's name with ``Error`` is recommended.
+
+Custom exceptions can be raised like any others, e.g.:
+
+.. code-block:: nim
+  raise newException(LoadError, "Failed to load data")
 
 Defer statement
 ---------------