refactor: PgArg to ProcParam

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.

src/PostgREST/DbStructure.hs

@@ -43,8 +43,9 @@ import Text.InterpolatedString.Perl6 (q)
 
 import PostgREST.DbStructure.Identifiers  (QualifiedIdentifier (..),
                                            Schema, TableName)
-import PostgREST.DbStructure.Proc         (PgArg (..), PgType (..),
+import PostgREST.DbStructure.Proc         (PgType (..),
                                            ProcDescription (..),
+                                           ProcParam (..),
                                            ProcVolatility (..),
                                            ProcsMap, RetType (..))
 import PostgREST.DbStructure.Relationship (Cardinality (..),
@@ -192,7 +193,7 @@ decodeProcs =
               <*> column HD.text
               <*> nullableColumn HD.text
               <*> compositeArrayColumn
-                  (PgArg
+                  (ProcParam
                   <$> compositeField HD.text
                   <*> compositeField HD.text
                   <*> compositeField HD.bool