Armor text change

[David Shaw <[e-mail]>]

Here's two suggested language changes that flow from the recent
discussion of armor:

*******

Section 6.2. Forming ASCII Armor has a sentence that reads:

   The header lines, therefore, MUST start at the beginning of a line, and
   MUST NOT have text following them on the same line.

Suggest to change it to:

   The header lines, therefore, MUST start at the beginning of a line, and
   MUST NOT have text other than whitespace following them on the same line.

(i.e. add "other than whitespace")

Ben Laurie pointed out the rationale that since the point of ignoring
trailing whitespace in armored and clearsigned messages was to work
around transport systems like mail that may add whitespace, then it is
necessary to ignore whitespace on the header lines as well.

*******

This other change I do not feel particularly strongly about.  It may
overspecify something that doesn't need it.  In section 6.2. Forming
ASCII Armor, add something to the effect of:

   Note that some transport methods are sensitive to line length.
   While there is a limit of 76 characters for the Radix-64 data
   (section 6.3), there is no limit to the length of Armor Headers.
   Care should be taken that the Armor Headers are short enough to
   survive transport.  One way to do this is to repeat an Armor Header
   key multiple times with different values for each so that no one
   line is overly long.

David