Documentation: highmem: use literal block for code example in highmem.h comment

When building htmldocs on Linus's tree, there are inline emphasis warnings
on include/linux/highmem.h:

Documentation/vm/highmem:166: ./include/linux/highmem.h:154: WARNING: Inline emphasis start-string without end-string.
Documentation/vm/highmem:166: ./include/linux/highmem.h:157: WARNING: Inline emphasis start-string without end-string.

These warnings above are due to comments in code example at the mentioned
lines above are enclosed by double dash (--), which confuses Sphinx as
inline markup delimiters instead.

Fix these warnings by indenting the code example with literal block
indentation and making the comments C comments.

Link: https://lkml.kernel.org/r/20220622084546.17745-1-bagasdotme@gmail.com
Fixes: 85a85e76

 ("Documentation/vm: move "Using kmap-atomic" to highmem.h")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Tested-by: Ira Weiny <ira.weiny@intel.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>