user-manual: fix rendering of history diagrams
Asciidoc appears to interpret a backslash at the end of a line as
escaping the end-of-line character, which screws up the display of
history diagrams like
o--o--o
\
o--...
The obvious fix (replacing "\" by "\\") doesn't work. The only
workaround I've found is to include all such diagrams in a LiteralBlock.
Asciidoc claims that should be equivalent to a literal paragraph, so I
don't understand why the difference--perhaps it's an asciidoc bug.
Cc: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>