docs: Makefile: Sort Documentation targets case-insensitively in make help

Avoid case-sensitive sorting when listing Documentation targets in make help.
Previously, targets like PCI and RCU appeared ahead of others due to uppercase
names.

Normalize casing during _SPHINXDIRS generation to ensure consistent and
intuitive ordering.

Fixes: 965fc39 ("Documentation: sort _SPHINXDIRS for 'make help'")
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Bhanu Seshu Kumar Valluri <bhanuseshukumar@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251104061723.16629-1-bhanuseshukumar@gmail.com>

Author: bhanuseshukumar

Documentation/Makefile

@@ -84,7 +84,7 @@ cleandocs:
 	$(Q)rm -rf $(BUILDDIR)
 
 # Used only on help
-_SPHINXDIRS   = $(sort $(patsubst $(srctree)/Documentation/%/index.rst,%,$(wildcard $(srctree)/Documentation/*/index.rst)))
+_SPHINXDIRS   = $(shell printf "%s\n" $(patsubst $(srctree)/Documentation/%/index.rst,%,$(wildcard $(srctree)/Documentation/*/index.rst)) | sort -f)
 
 dochelp:
 	@echo  ' Linux kernel internal documentation in different formats from ReST:'