Doc: Add how to change the notation fonts

Documentation/notation/text.itely

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 @c \version "2.21.0"
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 @c we don't do Helvetica / Courier, since GS incorrectly loads
 @c Apple TTF fonts
 
 The following syntax allows you to change the specific fonts,
 leaving others to the default values.
 The following example has the same effect
 as the above @code{make-pango-font-tree} example.
 The same as  @code{make-pango-font-tree},
 you can specify a comma-separated list of @q{fonts} for @code{roman},
-@code{sans}, and @code{typewriter} categoriles.

[Jean-Charles, 2017/08/26 08:16:26]

typo: categories

[trueroad, 2017/08/26 09:39:17]

Done.

+@code{sans}, and @code{typewriter} categories.
 If you don't change the staff size from the default of 20 pt,
 @code{#:factor (/ staff-height pt 20)} is unnecessary.
 
 @funindex set-global-fonts
 
 @example
 \paper @{
   #(define fonts
     (set-global-fonts
      #:roman "Times New Roman"
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 @end example
 
 @seealso
 Notation Reference:
 @ref{Fonts explained},
 @ref{Single entry fonts},
 @ref{Selecting font and font size},
 @ref{Font},
 @ref{Replacing the notation font}.