Fix, update, and improve client exceptions documentation (aio-libs#7733)

## What do these changes do?

In the *Hierarchy of exceptions* section of the *Client Reference*
documentation:
- Fix `ServerConnectionError` indentation, denoting that it subclasses
`ClientConnectionError`, not `ClientOSError`
- Fix `ServerFingerprintMismatch` indentation, denoting that it
subclasses `ServerConnectionError`, not `ClientOSError`
- Normalize the formatting so as to consistently use two, rather than
three, spaces for indentation
- Normalize the formatting so as to consistently include a blank newline
between exceptions
- Order exceptions alphabetically
- Add `TooManyRedirects`
- Add `UnixClientConnectorError`

Also order the documentation of client exceptions alphabetically by
hierarchy

CHANGES/7733.doc

@@ -0,0 +1 @@
+Fix, update, and improve client exceptions documentation.

docs/client_reference.rst

@@ -2138,13 +2138,6 @@ Response errors
       .. deprecated:: 3.1
 
 
-.. class:: WSServerHandshakeError
-
-   Web socket server response error.
-
-   Derived from :exc:`ClientResponseError`
-
-
 .. class:: ContentTypeError
 
    Invalid content type.
@@ -2165,6 +2158,13 @@ Response errors
 
    .. versionadded:: 3.2
 
+
+.. class:: WSServerHandshakeError
+
+   Web socket server response error.
+
+   Derived from :exc:`ClientResponseError`
+
 Connection errors
 ^^^^^^^^^^^^^^^^^
 
@@ -2191,14 +2191,6 @@ Connection errors
 
    Derived from :exc:`ClientConnectorError`
 
-.. class:: UnixClientConnectorError
-
-   Derived from :exc:`ClientConnectorError`
-
-.. class:: ServerConnectionError
-
-   Derived from :exc:`ClientConnectionError`
-
 .. class:: ClientSSLError
 
    Derived from :exc:`ClientConnectorError`
@@ -2215,6 +2207,14 @@ Connection errors
 
    Derived from :exc:`ClientSSLError` and :exc:`ssl.CertificateError`
 
+.. class:: UnixClientConnectorError
+
+   Derived from :exc:`ClientConnectorError`
+
+.. class:: ServerConnectionError
+
+   Derived from :exc:`ClientConnectionError`
+
 .. class:: ServerDisconnectedError
 
    Server disconnected.
@@ -2226,51 +2226,58 @@ Connection errors
       Partially parsed HTTP message (optional).
 
 
-.. class:: ServerTimeoutError
-
-   Server operation timeout: read timeout, etc.
-
-   Derived from :exc:`ServerConnectionError` and :exc:`asyncio.TimeoutError`
-
 .. class:: ServerFingerprintMismatch
 
    Server fingerprint mismatch.
 
    Derived from :exc:`ServerConnectionError`
 
+.. class:: ServerTimeoutError
+
+   Server operation timeout: read timeout, etc.
+
+   Derived from :exc:`ServerConnectionError` and :exc:`asyncio.TimeoutError`
+
 
 Hierarchy of exceptions
 ^^^^^^^^^^^^^^^^^^^^^^^
 
 * :exc:`ClientError`
 
-  * :exc:`ClientResponseError`
-
-    * :exc:`ContentTypeError`
-    * :exc:`WSServerHandshakeError`
-    * :exc:`~aiohttp.ClientHttpProxyError`
-
   * :exc:`ClientConnectionError`
 
     * :exc:`ClientOSError`
 
       * :exc:`ClientConnectorError`
 
-         * :exc:`ClientSSLError`
+        * :exc:`ClientProxyConnectionError`
+
+        * :exc:`ClientSSLError`
 
-           * :exc:`ClientConnectorCertificateError`
+          * :exc:`ClientConnectorCertificateError`
 
-           * :exc:`ClientConnectorSSLError`
+          * :exc:`ClientConnectorSSLError`
 
-         * :exc:`ClientProxyConnectionError`
+        * :exc:`UnixClientConnectorError`
 
-      * :exc:`ServerConnectionError`
+    * :exc:`ServerConnectionError`
 
-         * :exc:`ServerDisconnectedError`
-         * :exc:`ServerTimeoutError`
+      * :exc:`ServerDisconnectedError`
 
       * :exc:`ServerFingerprintMismatch`
 
+      * :exc:`ServerTimeoutError`
+
   * :exc:`ClientPayloadError`
 
+  * :exc:`ClientResponseError`
+
+    * :exc:`~aiohttp.ClientHttpProxyError`
+
+    * :exc:`ContentTypeError`
+
+    * :exc:`TooManyRedirects`
+
+    * :exc:`WSServerHandshakeError`
+
   * :exc:`InvalidURL`