update BooleanUtils.toBoolean(String str) javadoc to reflect changes 

the update to 2.4 to 2.6, particularly LANG-633 broke how BooleanUtils.toBoolean(String str) works.

apache 2.4 BooleanUtils.toBoolean("Y") = false 

apache 2.6 BooleanUtils.toBoolean("Y") = true 

this pull request updates the javadoc for BooleanUtils.toBoolean(String str)  and BooleanUtils.toBooleanObject(final String str)

basically these two method now accepts the following strings "y", "n", "t", "f" (case insensitive)