#5132 LM 2.4.1: fix errors in variable definitions

[Federico Bruni]

Reported almost 10 months ago:
http://lists.gnu.org/archive/html/bug-lilypond/2016-07/msg00072.html

I found a bug in the Learning Manual:
http://lilypond.org/doc/v2.19/Documentation/learning/organizing-pieces-with-variables.html

The example suggested at the end of that page doesn't work:

1. paperheight should be renamed to paper-height

2. name variable should be renamed to something else (myName), as it triggers this error:

warning: identifier name is a keyword: name'

1. width variable triggers this error:
   error: unknown escaped string: \cm'

I suggested to change it to:

myWidth = 60
myName = "Wendy"
aFivePaper = \paper { #(set-paper-size "a5") }

\paper {
  \aFivePaper
  line-width = \myWidth
}

{
  c'4^\myName
}

We may want to add that default unit of line-width value is \mm and that changing the unit in the variable definition is not allowed. See also this comment:
http://lists.gnu.org/archive/html/bug-lilypond/2016-07/msg00078.html

[Federico Bruni]

Doc: fix errors in variable definitions - issue #5132

http://codereview.appspot.com/325720043

[Anonymous]

Passes make, make check and a full make doc.

[Anonymous]

Patch on countdown for May 12th.

[Anonymous]

Patch counted down, please push.

[Federico Bruni]

pushed to staging as commit 250f1bf95f84616b68932324d6b5026833080cc4