[Rpc][update doc] apply rpc ppx syntax

[GuerfelMohamed]

No description provided.

[balat]

There are still many occurrences of server_function in tutorials. Do I need to do your documentation myself?
Also Eliom's documentation is not up to date.

tutos/dev/manual/how-to-call-a-server-side-function-from-client-side.wiki:let%client f_rpc = ~%(Eliom_client.server_function [%derive.json: int] f)
tutos/dev/manual/how-to-make-the-client-side-program-get-an-html-element-from-the-server-and-insert-it-in-the-page.wiki:  ~%(server_function [%derive.json: unit] get_mydiv)
tutos/dev/manual/how-to-send-a-file-to-server-without-stopping-the-client-process.wiki:there is a way to send it to the server. You can't use the {{{server_function}}} syntax for this, but you can use
tutos/dev/manual/tutoreact.wiki:use {{{server_function}}}:
tutos/dev/manual/tutoreact.wiki:  Eliom_client.server_function
tutos/dev/manual/tutoreact.wiki:  Eliom_client.server_function [%derive.json: int]
tutos/dev/manual/tutoreact.wiki:  Eliom_client.server_function
tutos/dev/manual/tutoreact.wiki:  Eliom_client.server_function
tutos/dev/manual/tutoreact.wiki:  Eliom_client.server_function [%derive.json: int]
tutos/dev/manual/tutoreact.wiki:  Eliom_client.server_function
tutos/dev/manual/tutoreact.wiki:  Eliom_client.server_function [%derive.json: int]
tutos/dev/manual/tutoreact.wiki:  Eliom_client.server_function [%derive.json: int]

[balat]

Do not merge this until:

• All occurrences of server_function in tuto are translated
• Eliom doc is updated
• OS template and doc are updated