Modify remove_unused_kernels text

This option was present in the docs but had the wrong default
value, and text that didn't match the code.

fixes bug 1153472

Typo fix (kernal > kernel) and some clarification on ramifications.

Change-Id: Id7f65cedc5da6526d4a5d4e969374d0b4e863b40
Merge "Modify remove_unused_kernels text"