documentation for PR #1071 - highlight criticalhp

Pull request from vultur-cadens:  add a new status highlight pattern
OPTIONS=hilite:hitpoints/criticalhp/color&attribute
to highlight hit points when they drop to the point where prayer
classifies them as a major problem.  The threshold for that varies
depending on max HP and experience level.

It affects hitpointbar as well as the status field for hit points.

Right now a highlight rule for HP down or HP changed or HP up takes
precedence over criticalhp until the temporary highlight times out.
I'm not sure how best to deal with that.  With regeneration and an
up or changed rule for HP, the criticalhp highlight probably won't be
seen even with a very short statushilites timeout.

I've added a fixes entry and updated Guidebook.mn but Guidebook.tex
is lagging.

Closes #1071

doc/Guidebook.mn

@@ -5424,6 +5424,11 @@ for stone through termill, \(lqminor_troubles\(rq for blind through hallu,
 Allowed behaviors are \(lqalways\(rq, \(lqup\(rq, \(lqdown\(rq,
 \(lqchanged\(rq, a percentage or absolute number threshold, or
 text to match against.
+For the \fIhitpoints\fP field, the additional behavior \(lqcriticalhp\(rq
+is available.
+It overrides always, percentage threshold, and absolute threshold if
+hit points are at or below the \fImajor problem\fP threshold
+(which varies depending upon maximum hit points and experience level).
 .si
 .lp "*"
 \(lqalways\(rq will set the default attributes for that field.
@@ -5479,6 +5484,10 @@ it also matches when value is below or above.
 If the prefix is \(oq<\(cq or \(oq>\(cq, only match when strictly
 above or below.
 .lp "*"
+criticalhp only applies to the hitpoints field and when defined it
+takes precedence over other hitpints rules except \fIchanged\fP,
+\fIup\fP, or \fIdown\fP.
+.lp "*"
 text match sets the attribute when the field value
 matches the text.
 Text matches can only be used for \(lqalignment\(rq,