doc: Add a comment to reset paper account when qty must be integer

[hiohiohio]

Fixes https://github.com/alpacahq/alpaca-py/issues/499

Context:

• To enable features recently added (i.e. fractional/options trading) for paper account, resetting the paper account is required.
  • this process is only required for old paper accounts not for recent paper accounts/live accounts

Changes:

• add a comment to reset paper account when facing an error of qty must be integer