Mention need to require autoloader in hooks using functions

[jeskew]

Add a line and example to the documentation covering the need to require the autoloader file in order to use autoloaded files.

Addresses #4184

[alcohol]

This will fail if autoload.php does not exist yet.

[jeskew]

@alcohol As we discussed in #4184, I believe the existing disclaimer about the state of the autoloader being undetermined in pre-install and pre-update commands covers the possible non-existence of vendor/autoload.php. I could repeat part of that disclaimer in the bullet point added in this PR if you believe it's unclear.

[alcohol]

Well at least set a good example :-(.
Don't use string interpolation, $vendorDir . '/autoload.php' is cleaner.

Also I think the current wording is a bit vague as to what kind of functions you are referring to. Remove the reiteration of the disclaimer (it's not necessary), and try to emphasise it's about functions that are normally only available via the files autoload attribute.

[jeskew]

What phrasing would you suggest is more clear than 'functions defined outside of a class'?

I've always found interpolation to be cleaner, and since there's no performance difference between interpolation and concatenation, we'll have to agree to disagree on the aesthetics.

[alcohol]

We can disagree on aesthetics, but we can agree on convention. So if you don't mind.

[alcohol]

As for phrasing, maybe it's better to turn it around and simply make it more clear that "must be autoloadable via Composer's autoload functionality" does not mean that the autoloader is actually loaded, but merely that the same mechanism is used and that this only applies to the psr-0, psr-4 and classmap autoload types, not files.

[jeskew]

@alcohol Were there any other changes you wanted to see made?

[alcohol]

Well,

If a defined callback relies on functions defined outside of a class, the callback itself is responsible for loading the appropriate files, as no files beyond those required for psr-0, psr-4, and classmap autoloading are loaded during Composer commands.

could probably better be phrased as

Callbacks can only autoload classes from psr-0, psr-4 and classmap definitions. If a defined callback relies on functions defined outside of a class, the callback itself is responsible for loading the file containing these functions.