Add note about sourceignore recursion

[kingdonb]

We do a recursive search for .sourceignore

https://github.com/fluxcd/pkg/blob/main/sourceignore/sourceignore.go#L114

    for _, fi := range fis {

With references: https://cloud-native.slack.com/archives/CLAJ40HV3/p1674742375696939?thread_ts=1674741099.305269&cid=CLAJ40HV3

Today we advised a user who was looking for another way to tell Flux to ignore a file in Git, and we found this fact about the recursion was not documented. I thought this was a common expectation, but I didn't want to reassure someone that it would work without testing to be sure, or a direct reference to that effect, so @stefanprodan found it for us 👍

Let's add a note about this to the docs, to save the trouble next time.

[hiddeco]

An OCIRepository also looks for ignore files:

source-controller/controllers/ocirepository_controller.go

Line 1023 in 5984c81

ps, err := sourceignore.LoadIgnorePatterns(dir, ignoreDomain)

[kingdonb]

It looks like whenever LoadIgnorePatterns is called, the recursive approach is being used. Thanks for the clarifications.

[kingdonb]

It also looks like the OCIRepository doc is missing a section that is targeted by the link text:

and may overrule the [`.sourceignore` file exclusions](#sourceignore-file)`

I'll add some text and make it aligned with the other docs.

[hiddeco]

Thank you @kingdonb 🙇