rebase docs: fix incorrect format of the section Behavioral Differences

rebase docs: fix incorrect format of the section Behavioral Differences

The text body of section Behavioral Differences is typeset as code,
but should be regular text. Remove the indentation to achieve that.

While here, prettify the language:

- use "the x backend" instead of "x-based rebase";
- use present tense instead of future tense;

and use subsections instead of a list.

Signed-off-by: Johannes Sixt <redacted>
Signed-off-by: Junio C Hamano <redacted>