manual/user guide/customization: expand introduction text

This patch expands the introduction text of chapter 'Project-specific
customization' by listing the type of customizations that can be performed
by a project.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>