docs(staking): add cancel-unbond delegations spec

[gsk967]

Description

Closes: #XXXX

related to #10885

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[amaury1093]

I think this should wait for #10885 first

[tac0turtle]

@AmauryM I think this is ready now?

[amaury1093]

lgtm, just some language changes

[atheeshp]

lgtm