ERL-1245: Improve documentation about file server in the file module

[OTP-Maintainer]

Original reporter: jeromedebretagne
Affected version: Not Specified
Component: kernel
Migrated from: https://bugs.erlang.org/browse/ERL-1245

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There is very little mention to the notion of "file server" in the [file|https://erlang.org/doc/man/file.html] module documentation while this seems to be currently an important implementation detail and even a requirement in many cases.

An additional intro about "file server" in [https://erlang.org/doc/man/file.html] would also be a great plus, as it seems to be a big differentiating feature whereas it's not clearly highlighted, only mentioned for a few functions (remote filer server, file server running as slave to the file server of another node, etc.).

I'm creating this issue as discussed in ERL-1244, since a PR would be welcome by the OTP team

Thanks, Jérôme