Improved the user-friendliness of errors shown by the summary feature in certain areas

Renamed format "signed decimal" to be "decimal". "unsigned decimal" remains unchanged:
 - the name "signed decimal" was interfering with symbol %S (use summary) in summary strings.
   because of the way summary strings are implemented, this did not really lead to a bug, but
   simply to performing more steps than necessary to display a summary. this is fixed.
Documentation improvements (more on synthetic children, some information on filters). This is still a WIP.

llvm-svn: 138384