Base: Document IPC syntax formally and informally

Author: https://github.com/kleinesfilmroellchen
Commit: https://github.com/SerenityOS/serenity/commit/489fce0ed2
Pull-request: https://github.com/SerenityOS/serenity/pull/20437
Reviewed-by: https://github.com/ADKaster ✅

Base/usr/share/man/man5/ipc.md

@@ -21,6 +21,66 @@ In other words, in case of the synchronous calls, the IPC library will not retur
 
 Ideally, all APIs for the server endpoint should be asynchronous.
 
+### Syntax Overview
+
+Each IPC endpoint definition has the form:
+
+```ipc
+endpoint MyServer
+{
+    // messages...
+}
+```
+
+You can use C++ `#include` directives before the `endpoint` keyword, which are copied to the resulting endpoint stub file. This is important if your messages use specific types of the library or application.
+
+Each message must appear on its own line. Synchronous messages are defined with an arrow `=>` like
+
+```ipc
+message_name(arguments...) => (return values...)
+```
+
+and asynchronous messages are defined with a "stopped arrow" `=|` like
+
+```ipc
+message_name(arguments...) =|
+```
+
+The argument and return value lists define what data the message passes to the other side, and what data will be retrieved back. There is no limitation to a single return value, since it will be converted to a `Messages` struct that can hold as much data as needed. The lists are defined like normal C++ formal parameter lists:
+
+```ipc
+message_name(int first_param, bool second_param, My::Library::Type third_param) => (Optional<int> first_return, float second_return)
+```
+
+Currently, parameter types cannot contain spaces, so be careful with templates.
+
+Parameters can contain attributes, which are a comma-separated list within a `[]` block preceding the type. The only currently implemented attribute is the `UTF8` attribute for string types, which will add a run-time validation that the string is valid UTF-8. For example:
+
+```ipc
+set_my_name([UTF8] String name) =|
+```
+
+For the String type in particular, this is not necessary.
+
+### Formal Syntax
+
+In Extended Backus-Naur form (and disregarding unwanted leniencies in the code generator's parser), IPC file syntax looks like the following:
+
+```ebnf
+IPCFile = { Endpoint } ;
+Endpoint = Includes, "endpoint", Identifier, "{", { Message }, "}" ;
+Identifier = (* C++ identifier *) ;
+Includes = { (* C++ preprocessor #include directive *) } ;
+
+Message = Identifier, "(", [ ParameterList ], ")", (SynchronousTrailer | AsynchronousTrailer) ;
+SynchronousTrailer = "=>", "(", [ ParameterList ], ")";
+AsynchronousTrailer = "=|" ;
+ParameterList = Parameter, { ",", Parameter } ;
+Parameter = [ "[", AttributeList, "]" ], TypeName, Identifier ;
+AttributeList = Identifier, { ",", Identifier } ;
+TypeName = (* C++ type name, without spaces *) ;
+```
+
 ## Examples
 
 To create a new connection, you first need to generate client and server endpoints.
@@ -28,7 +88,7 @@ These endpoints should implement the communication logic using the IPC compiler-
 
 Start from defining an endpoint in the IPC file in `MyServer.ipc`.
 
-```
+```ipc
 endpoint MyServer
 {
     SyncAPI(DeprecatedString text) => (i32 status)