regulator: Document "regulator-boot-on" binding more thoroughly

The description of "regulator-boot-on" was a little unclear, at least
to me.  Did this property mean that we should turn the regulator on at
boot?  Or perhaps it was intended only to be used for regulators where
we couldn't read the state at bootup to indicate what state we should
assume?  The answer, it turns out, is both [1].

Let's document this.

[1] https://lore.kernel.org/r/20190923181431.GU2036@sirena.org.ukSigned-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20191001124531.v2.1.Ice34ad5970a375c3c03cb15c3859b3ee501561bf@changeidSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/regulator.yaml

@@ -38,7 +38,12 @@ properties:
     type: boolean
 
   regulator-boot-on:
-    description: bootloader/firmware enabled regulator
+    description: bootloader/firmware enabled regulator.
+      It's expected that this regulator was left on by the bootloader.
+      If the bootloader didn't leave it on then OS should turn it on
+      at boot but shouldn't prevent it from being turned off later.
+      This property is intended to only be used for regulators where
+      software cannot read the state of the regulator.
     type: boolean
 
   regulator-allow-bypass: