nail.1: furtherly clarify doc. of \cX (Doug McIlroy)..

nail.1: furtherly clarify doc. of \cX (Doug McIlroy)..

Doug McIlroy pointed out at TUHS, where i impertinently
copied+pasted the -- then false -- \cX documentation:

 |Except for the ISO citations, this paragraph says the same
 |thing more succinctly.
 |
 |'\cX'    represents a nonprintable character Y in terms of the
 |          printable character X whose binary code is obtained
 |          by adding 0x40 (decimal 64) to that for Y. (In some
 |          historical contexts, '^' plays the role of '\c'.)
 |          Alternative standard representations for certain
 |          nonprinting characters, e.g. '\a', '\n', '\t' above,
 |          are preferred by S-nail. '\c@' (NUL) serves as a
 |          string terminator regardless of following characters.
 |
 |And this version, 1/3 the length of the original, tells all
 |one really needs to know.
 |
 |'\cX'    represents a nonprintable character Y in terms of the
 |          printable character X whose binary code is obtained
 |          by adding 0x40 (decimal 64) to that for Y. '\c@'
 |          (NUL) serves as a string terminator regardless of
 |          following characters.

This is not what i commit, since for my personal taste, at the
time of this writing, the above is too concise for many.  The
commit, and the respond that lingers in the other window, took an
entire afternoon.