help: limit documentation width to at most 80 characters

Reading long lines is suboptimal no matter how wide the terminal is.

mercurial/commands.py

@@ -1985,7 +1985,7 b' def help_(ui, name=None, with_version=Fa'
     Returns 0 if successful.
     """
     option_lists = []
-    textwidth = ui.termwidth() - 2
+    textwidth = min(ui.termwidth(), 80) - 2
 
     def addglobalopts(aliases):
         if ui.verbose: