thermal: Syntactic and factual errors in the API document

There are several places where the English in the document is syntactically
invalid, or unclear. There are also one or two factual errors.

Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Andy Champ <andycham@amazon.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>