improve doc-strings

The first sentence should summarize the variable's or function's
purpose and it should fit on the first line.  Change existing
doc-string by:

* Move first sentence onto first line even if that makes it _a bit_
  long.
* Move additional notes out of first sentence and add them later,
  possibly as complete sentences.
* If I am uncertain whether doing the above would alter the meaning,
  _don't_ do it.
* If fitting the initial sentence on the first line would require a
  complete rewrite of the doc-string _don't_ do so unless it is very
  easy to do.
* Remove indentation from second and later lines if it is there to
  align them with the first in the source code, instead of in
  `describe-*' output.
* Make "pullet point" lists a bit more consistent.

Obviously this does not fix all problems but it's a start.

mu4e/mu4e.el

@@ -29,7 +29,7 @@
 (eval-when-compile (require 'cl))
 
 (require 'mu4e-meta)     ;; autogenerated file with metadata (version etc.)
-(require 'mu4e-headers)     ;; headers view
+(require 'mu4e-headers)  ;; headers view
 (require 'mu4e-view)     ;; message view
 (require 'mu4e-main)     ;; main screen
 (require 'mu4e-compose)  ;; message composition / sending