docs/manual: get rid of legacy comment in GENDOC

Since dd935d6 (gendoc infra: use $(pkgname) instead of explicitly
passing 'manual', the GENDOC infra no longer expect any argument.

Remove the stray comment.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/manual.mk

@@ -124,9 +124,6 @@ endef
 ################################################################################
 # GENDOC -- generates the make targets needed to build asciidoc documentation.
 #
-#  argument 1 is the name of the document and must be a subdirectory of docs/;
-#             the top-level asciidoc file must have the same name
-#
 # The variable <DOCUMENT_NAME>_SOURCES defines the dependencies.
 ################################################################################
 define GENDOC