Explain better variable and constant declarations.
Leave one blank line between the rules for variable and constant declarations
(not necessary for ABNF, but consistent with the rest of the file and actually
expected by the ABNF-to-LaTeX converter).
Limit lines to 80 columns, by putting the rules for variable and constant
declarations over two lines each, with proper indentation.
By using markdown in the documentation comments of the grammar, the markdown
file generated from the grammar includes those markdown features in the text,
making it more readable and better-looking.
Also fixed a few typos in the documentation comments.
Also updated a few documentation comments that were out of date after making
changes to the grammar.
Also removed a now-obsolete grammar rule for "input" parameters of functions.
For both uniformity and clarity of reference from other documentation comments,
add some documentation comments next to every single character definition.
No change to the grammar itself.
