When sending an email manually so it can have multiple patches, point out

When sending an email manually so it can have multiple patches, point out
that Mutt can construct the email for you.

Obviously this isn't as generally relevant as the rest of the Guide, since
patchers will use many different mail clients. But it's a significant boon
for those who do use Mutt, and a very short addition to the Guide.

Mutt is singled out simply because it has this functionality; I suspect
that most other widely used mail clients don't.

Committer: Removed trailing whitespace.  Applied patch manually because other
lines in the file had been rebroken and patch no longer applied cleanly.

For: RT #119599