kconfig-language: improve menuconfig usage description

Improper menuconfig usage leads to empty menu entries.
zconfdump() is able to reveal some real-life examples:
- menuconfig VFIO_NOIOMMU
- menuconfig RESET_CONTROLLER
- menuconfig SND_ARM

To avoid future occurrences of those, improve the menuconfig syntax
description.

Signed-off-by: Eugeniu Rosca <rosca.eugeniu@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kbuild/kconfig-language.txt

@@ -274,7 +274,44 @@ menuconfig:
 
 This is similar to the simple config entry above, but it also gives a
 hint to front ends, that all suboptions should be displayed as a
-separate list of options.
+separate list of options. To make sure all the suboptions will really
+show up under the menuconfig entry and not outside of it, every item
+from the <config options> list must depend on the menuconfig symbol.
+In practice, this is achieved by using one of the next two constructs:
+
+(1):
+menuconfig M
+if M
+    config C1
+    config C2
+endif
+
+(2):
+menuconfig M
+config C1
+    depends on M
+config C2
+    depends on M
+
+In the following examples (3) and (4), C1 and C2 still have the M
+dependency, but will not appear under menuconfig M anymore, because
+of C0, which doesn't depend on M:
+
+(3):
+menuconfig M
+    config C0
+if M
+    config C1
+    config C2
+endif
+
+(4):
+menuconfig M
+config C0
+config C1
+    depends on M
+config C2
+    depends on M
 
 choices: