internal/secret: add FlagVar
internal/secret: add FlagVar

The standard flag package offers both String and StringVar API flavors,
and each one is sometimes more convenient than the other (depending on
the situation). Offer both Flag and FlagVar variants in secret as well.

Also reword our documentation slightly for consistency with flag's own.

Change-Id: I2996c1df25c62fa6e01fbf36e6d952a8f5337811
Reviewed-on: https://go-review.googlesource.com/c/build/+/411574
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

golang/build,golang/build,golang/build,golang/build