kbuild: update comment block of scripts/clang-version.sh

Commit 469cb737 ("kconfig: add CC_IS_CLANG and CLANG_VERSION")
changed the code, but missed to update the comment block.

The -p option was gone, and the output is 5-digit (or 6-digit when
Clang 10 is released).

Update the comment now.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/clang-version.sh

 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 #
-# clang-version [-p] clang-command
-#
-# Prints the compiler version of `clang-command' in a canonical 4-digit form
-# such as `0500' for clang-5.0 etc.
-#
-# With the -p option, prints the patchlevel as well, for example `050001' for
-# clang-5.0.1 etc.
+# clang-version clang-command
 #
+# Print the compiler version of `clang-command' in a 5 or 6-digit form
+# such as `50001' for clang-5.0.1 etc.
 
 compiler="$*"