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>

include/linux/highmem.h

@@ -149,18 +149,18 @@ static inline void *kmap_local_folio(struct folio *folio, size_t offset);
  * It is used in atomic context when code wants to access the contents of a
  * page that might be allocated from high memory (see __GFP_HIGHMEM), for
  * example a page in the pagecache.  The API has two functions, and they
- * can be used in a manner similar to the following:
+ * can be used in a manner similar to the following::
  *
- * -- Find the page of interest. --
+ *   // Find the page of interest.
  *   struct page *page = find_get_page(mapping, offset);
  *
- * -- Gain access to the contents of that page. --
+ *   // Gain access to the contents of that page.
  *   void *vaddr = kmap_atomic(page);
  *
- * -- Do something to the contents of that page. --
+ *   // Do something to the contents of that page.
  *   memset(vaddr, 0, PAGE_SIZE);
  *
- * -- Unmap that page. --
+ *   // Unmap that page.
  *   kunmap_atomic(vaddr);
  *
  * Note that the kunmap_atomic() call takes the result of the kmap_atomic()