Document how opam chooses the default compiler during opam init and opam switch create

[kit-ty-kate]

Fixes #6407

[dra27]

Perhaps the FAQ isn't the correct place to be noting opam switch create 5.4 ocaml.5.4.0, but it would go well with the part about opam init --compiler (i.e. that you can just specify the precise version package and not rely on any of these heuristics)

[dra27]

Suggested change

	Since opam 2.4, `opam init` will choose `ocaml-base-compiler` as default
	compiler for the `default` switch, and let the solver choose the latest
	version. Of course this default can always be changed using `--compiler`
	or by skipping the creation of the `default` switch all-together with
	`--bare`.
	Since opam 2.4, `opam init` will select `ocaml-base-compiler` as the default
	compiler package for the `default` switch, and let the solver choose the latest
	version. This can always be changed using `--compiler` or by skipping the
	creation of the `default` switch altogether with `--bare`.

[dra27]

Suggested change

	installed by other means, you can use the `ocaml-system` package as follows:
	installed outside opam, you can use the `ocaml-system` package as follows:

?

[dra27]

Suggested change

	For example if you would rather use the OCaml compiler that you have
	installed by other means, you can use the `ocaml-system` package as follows:
	`opam init --compiler=ocaml-system`. Be careful though, system-wide compilers
	can be setup in a way that the default opam-repository doesn't expect,
	so failures may occur. Thus this option should only be used if you know
	what you're doing.
	For example, if you would rather use the OCaml compiler that you have
	installed by other means, you can use the `ocaml-system` package as follows:
	`opam init --compiler=ocaml-system`. Be careful, though; system-wide compilers
	can be set-up in a way that the default opam-repository doesn't expect,
	so failures may occur. Thus this option should only be used if you know
	what you're doing.

[dra27]

Suggested change

	so failures may occur. Thus this option should only be used if you know
	so failures may occur. We recommend this option should only be used if you know

?

[dra27]

Suggested change

	`opam switch create <version>` on the other hand, works slightly differently.
	Since the only unknown is: which package is going to be used.
	`opam switch create <version>` on the other hand, works slightly differently,
	since the only unknown is exactly which package is going to be used.

[dra27]

Suggested change

	Opam decides this by looking at all the packages that have been tagged as
	`compiler`, filter which ones have the right version, and then let the solver
	decide. For example calling `opam switch create 5.4 5.4.0` will create a
	switch named `5.4` and look for compiler packages with versions `5.4.0` which
	by default, with [opam-repository](https://github.com/ocaml/opam-repository/),
	will get back with `ocaml-base-compiler.5.4.0` and `ocaml-system.5.4.0`.
	Since 2025, `ocaml-system` is tagged as `avoid-version` so the solver will
	choose `ocaml-base-compiler.5.4.0`.
	Opam decides this by finding all packages which have the given and which
	that have been flagged as `compiler` and then presents just these packages
	to the solver. For example calling `opam switch create 5.4 5.4.0` will create a
	switch named `5.4` and look for compiler packages with versions `5.4.0` which
	by default, with [opam-repository](https://github.com/ocaml/opam-repository/),
	will be with `ocaml-base-compiler.5.4.0`, `ocaml-system.5.4.0` and
	`ocaml-variants.5.4.0+options`. `ocaml-system.5.4.0` is flagged as `avoid-version`,
	so the solver will choose `ocaml-base-compiler.5.4.0`.

(although I can't remember for certain why ocaml-base-compiler.5.4.0 is guaranteed to be selected, given that it actually has more dependencies than ocaml-variants.5.4.0+options)