For POST on RPC, allows:
* passing a json object without using `Prefer: params=single-object`
The function must be defined with a single unnamed json param and
`Content-Type: application/json` must be specified.
* uploading binary to a function
The function must be defined with a single unnamed bytea param and
`Content-Type: application/octet-stream` must be specified.
* uploading raw text to a function
The function must be defined with a single unnamed text param and
`Content-Type: text/plain` must be specified.
BREAKING CHANGE If there's a function "my_func" having a single
unnamed json param and other overloaded pairs(with any number of
params), PostgREST won't be able to resolve a POST request to
"my_func". For solving this, you can name the unnamed json param.
my_func(json) -> my_func(prm json)
Clarify the difference between arguments and parameters.
Parameters are part of the function definition, arguments are the values
passed to the function.
Also clarify the findProc function comments and error message.
steve-chavez