[4.x] Clarify default field instructions/helper-text

[caseydwyer]

This tiny tweak originated in a Discord chat with Erin & Sam. Currently, the fields_default_instructions—which is used for the helper text below Default Value on field definitions—reads as:

Set the default value.

This PR tries to put a lil more meat on dem bones. 🍖 Proposing a change to:

Set a default value. Inserted whenever this field is empty, on all publishable forms.

This is definitely a subjective thing, so won't hurt my feelings if you all decide to reject or edit...but this has bit me a couple of times, with how the setting actually works. Copying in a bit of that Discord convo linked above, for more context:

I tend to think of default as more of a UI consideration—ie, a default value to save the user some time in the GUI, by pre-populating the field at time of creation, with a commonly-used default. And what's actually happening, I would maybe consider more of a fallback that's typically handled via code—ie, a stand-in value, in the event that the user hasn't provided a value and the system needs one, for whatever reason.

The way Statamic actually works is kind of an interesting hybrid, IMHO...it's similar to the former, as it's purely a front-end thing (at least as far as I can tell...nothing seems to be happening server-side to "force" that default into place). But it has the persistence of the latter, as it always attempts to "re-insert" the default, including on edit forms.

So as an example, say a user goes to create a new entry and clears the default value from the field. This saves as expected, with a null value. But if they return to edit that previously-saved entry—ie, any one that has a blank value in the field—the default value will be "quietly" re-entered via the CP GUI. And they might not realize that, causing confusion and/or problems when they unknowingly save a value into that field.

Again, totally realize this is mostly subjective semantics. TBH, would be nice to have both a default and a fallback to serve both purposes. But no biggie—just thought it might be beneficial for all, to have a slightly more descriptive rundown on how that works under the hood.

[caseydwyer]

Man just re-read that, and lol at me—context is about 100x as long as the actual code edits. 😂
Whoever gets stuck handling this PR, probably:

[jasonvarga]

It was a good read. Thanks!

[caseydwyer]

Haha, no problem—thanks Jason!