docs: make sure literal "->" isn't converted to arrow

docs: make sure literal "->" isn't converted to arrow

Recent versions of asciidoc will treat "->" as a
single-glyph arrow symbol, unless it is inside a literal
code block. This is a problem if we are discussing literal
output and want to show the ASCII characters.

Our usage falls into three categories:

  1. Inside a code block. These can be left as-is.

  2. Discussing literal output or code, but inside a
     paragraph. This patch escapes these as "\->".

  3. Using the arrow as a symbolic element, such as "use the
     Edit->Account Settings menu". In this case, the
     arrow symbol is preferable, so we leave it as-is.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>