doc options: state that all |-separated tuples need escaping

For the "completions" option type, the documentation states that |
and \ need to be escaped as \| and \\.
The same parser is for other option types that are lists-of-tuples:
range-specs and line-specs, so they need escaping too. Document that.

Only their last element can contain arbitrary data, so range-specs
and line-specs could work without escaping if we tweaked the parser.

doc/pages/options.asciidoc

@@ -114,7 +114,7 @@ are exclusively available to built-in options.
           is empty, but still valid.
 
         * _string_ is an arbitrary string which is associated with
-          the range.
+          the range. Any `|` or `\` characters must be escaped as `\|` or `\\`.
 
     All numeric fields are 1-based.
 
@@ -138,6 +138,9 @@ are exclusively available to built-in options.
     `set -add` appends the new specs to the list. +
     `set -remove` removes the given specs from the list. +
 
+    Any `|` or `\` characters that occur within `<flag text>` must be
+    escaped as `\|` or `\\`.
+
 *completions*::
     a list of `<text>|<select cmd>|<menu text>` candidates,
     except for the first element which follows the