Rectification in account_lines docs

[ckeshava]

1. "Possible Errors" description: actMalformed is returned if the account-number cannot be parsed properly. It has been incorrectly attributed to the marker field in the docs.

2. Inclusion of ignore_default request field: The documentation does not make a mention of the ignore_default field in the account_lines request. Internal functions inside rippled allow for this input, as explained in this PR: https://github.com/ripple/rippled/pull/3980