manual: document dependencies on atomic operations

Add atomic operations to the list of generic dependencies.

Since this is an architecture option, there is no comment string to be
added.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
[yann.morin.1998@free.fr: use the new arch-option; remove comment string]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/adding-packages-directory.txt

@@ -219,6 +219,11 @@ use in the comment.
 ** Dependency symbol: +BR2_USE_MMU+
 ** Comment string: no comment to be added
 
+* Atomic instructions (whereby the architecture has instructions to
+  perform some operations atomically, like LOCKCMPXCHG on x86)
+** Dependency symbol: +BR2_ARCH_HAS_ATOMICS+
+** Comment string: no comment to be added
+
 * Kernel headers
 ** Dependency symbol: +BR2_TOOLCHAIN_HEADERS_AT_LEAST_X_Y+, (replace
    +X_Y+ with the proper version, see +toolchain/toolchain-common.in+)