doc: update rzshell page (#5040)

Co-authored-by: Anton Kochkov <anton.kochkov@gmail.com>

doc/rzshell.md

@@ -1,44 +1,11 @@
 # Command parsing and command handling
 
-Rizin has moved away from the default way of parsing radare2 commands and the
-way commands were handled there. It enables by default what is/was called in r2
-`cfg.newshell`, which enables a generated parser that parses rizin commands and
-a new way of registering and developing commands.
-
-## A bit of history
-
-Rizin is a fork of radare2. Radare2 did not have, until recently, a generic
-parser for user inputs, but each command had to parse its arguments by itself.
-Moreover, there was no global register of commands available in the radare2
-shell, instead the input was chopped by looking for specific characters and then
-it was analyzed char by char, using big switch-cases to recognize the right
-command.
-
-As an example, you can see
-[cmd_flag.c:1163](https://github.com/rizinorg/rizin/blob/cde558e6e5788d0a6d544ab975b144ed59190676/librz/core/cmd_flag.c#L1163),
-which identifies the `fsr` command and then parses its input to check if an
-argument was available or not.
-
-This approach, although simple at the beginning, has some drawbacks like the
-inconsistency coming from having many different places in the code doing mostly
-the same thing (e.g. checking if an argument is available or not), the inability
-to easily register/unregister new commands at runtime (e.g. a new Core plugin
-that wants to provide a new command) or the inconsistency between commands
-actually available and commands shown to users in help messages.
-
-## Rizin shell
-
-Not long ago, radare2 introduced the variable `cfg.newshell` that, when enabled,
-allows you to use new features in the code. Rizin has chosen to enable this by
-default and it is going to transition most commands to the new way of writing
-commands, which will make it easier/faster to write commands and make the
-overall CLI experience more consistent and reliable.
-
-Rizin uses a parser generated with
+The Rizin shell language is originally derived from the Radare2 shell language.
+There is a parser generated with
 [tree-sitter](https://tree-sitter.github.io/tree-sitter/), which allows you to
 write grammars in JavaScript. You can see our grammar
 [here](https://github.com/rizinorg/rizin/blob/dev/subprojects/rizin-shell-parser/grammar.js).
-The parser recognizes the entire syntax of the rizin/radare2 shell language,
+The parser recognizes the entire syntax of the rizin shell language,
 like:
 
 - [basic statements](https://github.com/rizinorg/rizin/blob/6f40dfe493f0caf9e0541e1ee83e3d8012b5750f/subprojects/rizin-shell-parser/grammar.js#L238): `<command-name> <arg1> <arg2> ... <argN>`
@@ -48,7 +15,7 @@ like:
 - [grep statements](https://github.com/rizinorg/rizin/blob/6f40dfe493f0caf9e0541e1ee83e3d8012b5750f/subprojects/rizin-shell-parser/grammar.js#L148): `<statement>~<grep-pattern>`
 - and many others
 
-These patterns deal with the structure of the rizin/radare2 shell language, but
+These patterns deal with the structure of the rizin shell language, but
 they don't parse the input of each specific command available in the rizin shell
 (e.g. `af`, `pd`, etc.). The parser just splits the input statement into a
 "command name" and a list of "arguments".
@@ -70,7 +37,7 @@ deregister it, call the right command descriptor handler based on a list of
 command name + arguments, get the help of a command and potentially do many
 other things.
 
-As rizin/radare2 commands mainly form a tree, `RzCmdDesc` are organized in a
+As rizin commands mainly form a tree, `RzCmdDesc` are organized in a
 tree, with each descriptor having references to its parent and its children.
 Moreover, a descriptor has its help messages and its handler.