doc(rpc): Request Types -> Request Methods & reword description

The POST description was ridiculously confusing. This cleans up the
doc a bit and is hopefully a bit more straight-forward.

Signed-off-by: Kevin Morris <kevr@0cost.org>

doc/rpc.txt

@@ -28,21 +28,27 @@ Package information can be obtained by issuing HTTP GET requests of the form
 +/rpc?v=5&type=info&arg[]=_pkg1_&arg[]=_pkg2_&...+ where _pkg1_, _pkg2_, ...
 are the names of packages to retrieve package details for.
 
-Request Types
--------------
+Request Methods
+---------------
 
-The GET method here parses arguments in an odd way due to `v=5` historically
-supporting this ordering. Later versions will remove support for this kind
-of parsing, and the POST method is our first step toward solving it.
+Historically, the `type=multiinfo` `v=5` GET request has supported a
+particular ordering of arguments. The POST request argument ordering
+cannot be guaranteed, and so its behavior is different. Differences are
+described below:
 
-* `GET`
-    - Search arguments are constructed using the last found argument(s).
-    If the last related argument is `arg[]`, we collect arguments from
-    end to start until we hit a non-`arg[]` argument. If the last related
-    argument is `arg`, it used as the one and only argument.
-* `POST`
-    - Search arguments are constructed using `[arg] + args` where
-    `args == arg[]`.
+`GET`::
+  `type=multiinfo` arguments are parsed by iterating the query string
+  from last to first key, looking for an `arg` or `arg[]`. Once one is
+  found, behavior diverges depending on which is found first: the `arg`
+  parameter is used as the sole argument or the `arg[]` parameters are
+  built into a list until a non-argument key is encountered.
+`POST (experimental)`::
+  All provided instances of `arg` and `arg[]` given to `type=multiinfo`
+  are supported in unison:
+
+  curl -d 'v=5' -d 'type=info' -d 'arg=one' -d 'arg[]=two' -d 'arg[]=three' ...
+
+All other valid query types are supported without change.
 
 Examples
 --------