docs/manual: add musl to toolchain dependency options

Currently, we only document (e)glibc and uClibc as C libraries on which
we can depend. However, we've supported musl for quite some time now.

So, also refference it in the manual, when dealing with toolchain
options a package may have to depend on.

[Peter: order musl before uClibc]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/adding-packages-directory.txt

@@ -232,7 +232,7 @@ use in the comment.
 
 * C library
 ** Dependency symbol: +BR2_TOOLCHAIN_USES_GLIBC+,
-   +BR2_TOOLCHAIN_USES_UCLIBC+
+   +BR2_TOOLCHAIN_USES_MUSL+, +BR2_TOOLCHAIN_USES_UCLIBC+
 ** Comment string: for the C library, a slightly different comment text
    is used: +foo needs an (e)glibc toolchain+, or `foo needs an (e)glibc
    toolchain w/ C++`