Please document alternativeRestores context property

[address@hidden]

Please document the alternativeRestores context property, including its default 
setting. The default settings can lead to apparent bar-check failures when 
there are time signature changes in an \alternative block.
Thanks, 
Holland 

The default is: alternativeRestores = #'(measurePosition measureLength 
lastChord)

\version "2.19.49"

music =
 \repeat volta 2 {
       c1
 }
 \alternative {
   {
     \time 3/4
     d2. |
     \time 4/4
     c1 |
   }
   { d1 | }
 }

<<
 \relative c'' \music
 \relative c' \music
>>