std.math.big.int.Managed: adjust size of arg for limbs_buffer in format()

[poypoyan]

Fixes: #19787

Error occurs because in the current code, the size of limbs (hard-coded to 128) can be less than the calcToStringLimbsBufferLen, which is the minimum according to toString() documentation.

Also updated the doc and added tests for "(BigInt)" printing.