doc: avoid confusing K argument in the man pages of head and tail

doc: avoid confusing K argument in the man pages of head and tail

The character 'K' representing numeric user input can be confusing,
especially in the context where it can be suffixed by the multipliers
K, M, G, etc.
Use NUM instead.

* doc/coreutils.texi (head invocation): Replace @var{k} by @var{num}.
Also change @var{count} in the paragraph about the obsolete option
syntax.
(tail invocation): Likewise.  Furthermore, adjust the option order to
alphabetical sorting.
* src/head.c (usage): s/K/NUM/ in as placeholder in the -c and -n
options.  Furthermore, print the actual DEFAULT_NUMBER instead of the
hard-coded 10.
* src/tail.c (usage): s/K/NUM/ in as placeholder in the -c and -n
options.  Add "[+]" to clarify that NUM may optionally be preceded by
this sign.  Remove the redundant explanation of the optional '+' prefix
for NUM, as that is now clear from the option description above.

Suggested by Dan Jacobson in http://bugs.gnu.org/22042