Install libgtksourceview2.0-doc by entering the following commands in the terminal:
sudo apt update sudo apt install libgtksourceview2.0-doc
Description:
documentation for the GTK+ syntax highlighting widget
GtkSourceView is a text widget that extends the standard GTK+ 2.x text widget GtkTextView. It improves GtkTextView by implementing syntax highlighting and other features typical of a source editor. . This package contains the GtkSourceView reference manual.
Homepage: http://projects.gnome.org/gtksourceview/
Version: 2.10.5-3
Section: universe/doc