Handle non-integer builder.boostFactor

[wemeetagain]

We should handle non-integer builder boost factor especially considering our docs reference the formula that can easily result in a non-integer output.

Possible solutions:

• round or truncate to integer value, possibly w/ warn log
• throw friendly error and shutdown, include documentation update to highlight that

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Looks good to me now, applied suggestions from @g11tech and fixed example to use an integer value as --builder.boostFactor does not support floating point numbers

Originally posted by @nflaig in #6295 (review)

[philknows]

Should probably do this for the next release for the full feature to be implemented properly.