Clarify in sendmany/z_sendmany rpc docs that amounts are not FP
Fixes#3539
I clarified in the docs that only 8 digits of precision are allowed. For example, if you try 9 digits you will get:
```
zcash-cli z_sendmany $TADDR "[{\"address\":\"$ZADDR\",\"amount\":0.123456789}]"
error code: -3
error message:
Invalid amount
```