immediate operands without dollar sign for inline asm

[Luchezar Belev]

Hello,
Sometimes one needs to pass an immediate operand to asm statement that
should not be prefixed with dollar sign (e.g. for doing custom things
with assembly directives).

After much search in the internet i managed to find the way to do it:
when the operand is used in the assembly string, it must have the 'a'
modifier. (e.g. ".equ name, %a0" instead of ".equ name, %0".

My question is: why is this feature not mentioned anywhere in the GCC
documentation and is so extremely hard to find info about? Does this
mean that it is planned for removal or deprecation? Can one rely on
it's presence in future GCC versions?
(I think it is extremely useful for some specific tasks, please don't
remove it!)