whatsnew: http.server send_error explain parameter.

Also rewrote the send_error description for clarity and correct English.

Doc/library/http.server.rst

@@ -116,7 +116,7 @@ of which this module provides three different variants:
       HTTP error code value. *message* should be a string containing a
       (detailed) error message of what occurred, and *explain* should be an
       explanation of the error code number. Default *message* and *explain*
-      values can found in the *responses* class variable.
+      values can found in the :attr:`responses` class variable.
 
    .. attribute:: error_content_type
 
@@ -173,11 +173,14 @@ of which this module provides three different variants:
    .. method:: send_error(code, message=None, explain=None)
 
       Sends and logs a complete error reply to the client. The numeric *code*
-      specifies the HTTP error code, with *message* as optional, more specific
-      text, usually referring to short message response.  The *explain*
-      argument can be used to send a detailed information about the error in
-      response content body.  A complete set of headers is sent, followed by
-      text composed using the :attr:`error_message_format` class variable.
+      specifies the HTTP error code, with *message* as an optional, short, human
+      readable description of the error.  The *explain* argument can be used to
+      provide more detailed information about the error; it will be formatted
+      using the :attr:`error_message_format` class variable and emitted, after
+      a complete set of headers, as the response body.  The :attr:`responses`
+      class variable holds the default values for *message* and *explain* that
+      will be used if no value is provided; for unknown codes the default value
+      for both is the string ``???``.
 
       .. versionchanged:: 3.4
          The error response includes a Content-Length header.

Doc/whatsnew/3.4.rst

@@ -684,6 +684,18 @@ The *strict* argument of :class:`~html.parser.HTMLParser` is now deprecated.
 (Contributed by Ezio Melotti in :issue:`15114`)
 
 
+http
+----
+
+:meth:`~http.server.BaseHTTPRequestHandler.send_error` now accepts an
+optional additional *exaplain* parameter which can be used to provide an
+extended error description, overriding the hardcoded default if there is one.
+This extended error description will be formatted using the
+:attr:`~http.server.HTTP.error_message_format` attribute and sent as the body
+of the error response.  (Contributed by Karl Cow in :issue:`12921`.)
+
+
+
 importlib
 ---------