Add docs for the LAYOUT_* macro to layouts

[evantravers]

When moving my planck layout to my let's split, I ran into this issue
because I had copied my planck layout from the default layout which just
used the {} array form for the keymap layers. I checked the docs, but
this bit wasn't clear to me. I'm sure @ishtob or @drashna helped me on
discord, but this seemed to be the logical place to add a helpful hint.

I'm not sure whether my technical lingo is correct here: Is the
LAYOUT_ortho_4x12 really a macro? Any other changes?

[noroadsleft]

LAYOUT_ortho_4x12 really is a macro, though the way it works varies by keyboard.

Most 4x12 ortho keyboards define it "directly" somewhere, like the NIU Mini:

qmk_firmware/keyboards/niu_mini/niu_mini.h

Lines 19 to 30 in c63d9ee

	#define LAYOUT_ortho_4x12( \
	k00, k01, k02, k03, k04, k05, k06, k07, k08, k09, k0a, k0b, \
	k10, k11, k12, k13, k14, k15, k16, k17, k18, k19, k1a, k1b, \
	k20, k21, k22, k23, k24, k25, k26, k27, k28, k29, k2a, k2b, \
	k30, k31, k32, k33, k34, k35, k36, k37, k38, k39, k3a, k3b \
	) \
	{ \
	{ k00, k01, k02, k03, k04, k05, k06, k07, k08, k09, k0a, k0b }, \
	{ k10, k11, k12, k13, k14, k15, k16, k17, k18, k19, k1a, k1b }, \
	{ k20, k21, k22, k23, k24, k25, k26, k27, k28, k29, k2a, k2b }, \
	{ k30, k31, k32, k33, k34, k35, k36, k37, k38, k39, k3a, k3b } \
	}

In the case of the Planck however, it currently works a bit differently.

Planck standard for a while has been to use a macro named LAYOUT_planck_grid, which is functionally identical to LAYOUT_ortho_4x12. Before that, it used the C array syntax; indeed, a lot of the Planck keymaps still do it that way. There is the possibility that the Planck macro naming scheme changes in the near future, as @jackhumbert wants a consistent standard between Planck, Preonic and the upcoming Subatomic (which will be 5x14).

The Planck currently defines LAYOUT_planck_grid on lines 20-31 of planck.h for revisions 1 through 5, and on lines 121-136 for rev6 boards. These macros are aliased as LAYOUT_ortho_4x12 on lines 48 and 139, respectively.

For the Planck, that means that a keymap can use LAYOUT_planck_grid or LAYOUT_ortho_4x12, and the end result is exactly the same.

As far as your lingo, I see no problems, though someone else may have a different preference.

I do have a review comment for you, but it's unrelated to anything you've done here; it's more of a "since you're here, could you fix this thing that was already messed up?" issue.

[noroadsleft]

And subsequently, I learned you can't leave review comments about lines that haven't been edited in the PR.

Anyway, on Line 26, could you change

(`[a-z0-9_]`)

to

(`[a-z0-9_]+`)

? When I was proof-reading the document, I noticed this regex was wrong.

[drashna]

Thanks!