Fix small typo in fungible token freeze docs (#4943)

Hi!

In the course of a talk with @shawntabrizi

 in Singapore, we realized the
documentation related to freeze balances' a little bit confusing. It
stated that a frozen amount is released at some specified block number,
which isn't true in general.

This PR fixes that typo and further specifies that the frozen balance
may exceed the available balance, according to what we learned at the
PBA. This feature was not specified in the documentation AFAIK.

This is the first time I submit something to the polkadot SDK repo, so
please feel free to rephrase the docs I added in case I messed up!

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Co-authored-by: Shawn Tabrizi <[email protected]>
Co-authored-by: command-bot <>