doc: improve various BLOCKSIZE and SIZE help

doc: improve various BLOCKSIZE and SIZE help

* doc/coreutils.texi (multiplierSuffixes): Mention that
the suffix can be specified without a leading number
* src/split.c (usage): Refactor SIZE help to within a function
* src/truncate.c (usage): Likewise
* src/ls.c (usage): Likewise
* src/df.c (usage): Likewise. Also add a function with BLOCKSIZE help
* src/du.c (usage): Likewise.
* src/system.h: Define 2 functions to emit common help text
This was prompted by https://bugzilla.redhat.com/show_bug.cgi?id=511188