manual: update Config.in dependency explainations

--------------------------

Note that these two dependency types are only transitive with the

dependencies of the same kind.

This means, in the following example:

--------------------------

config BR2_PACKAGE_A

        bool "Package A"

config BR2_PACKAGE_B

        bool "Package B"

        depends on BR2_PACKAGE_A

config BR2_PACKAGE_C

        bool "Package C"

        depends on BR2_PACKAGE_B

config BR2_PACKAGE_D

        bool "Package D"

        select BR2_PACKAGE_B

config BR2_PACKAGE_E

        bool "Package E"

        select BR2_PACKAGE_D

--------------------------

* Selecting +Package C+ will be visible if +Package B+ has been

  selected, which in turn is only visible if +Package A+ has been

  selected.

* Selecting +Package E+ will select +Package D+, which will select

  +Package B+, it will not check for the dependencies of +Package B+,

  so it will not select +Package A+.

* Since +Package B+ is selected but +Package A+ is not, this violates

  the dependency of +Package B+ on +Package A+.  Therefore, in such a

  situation, the transitive dependency has to be added explicitly:

--------------------------

config BR2_PACKAGE_D

	bool "Package D"

	select BR2_PACKAGE_B

	depends on BR2_PACKAGE_A

config BR2_PACKAGE_E

	bool "Package E"

	select BR2_PACKAGE_D

	depends on BR2_PACKAGE_A

--------------------------

Overall, for package library dependencies, +select+ should be

preferred.

Note that such dependencies will make sure that the dependency option

is also enabled, but not necessarily built before your package. To do

so, the dependency also needs to be expressed in the +.mk+ file of the