string: Add missing kernel-doc return descriptions

While running kernel-doc validation on linux-next, warnings were emitted
for functions in include/linux/string.h due to missing return value
documentation:

    Warning: include/linux/string.h:375 No description found for return value of 'kbasename'
    Warning: include/linux/string.h:560 No description found for return value of 'strstarts'

This patch adds the missing return value descriptions for both functions
and clears the related kernel-doc warnings.

Signed-off-by: Kriish Sharma <kriish.sharma2006@gmail.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://patch.msgid.link/20251118184828.2621595-1-kriish.sharma2006@gmail.com
Signed-off-by: Kees Cook <kees@kernel.org>

Author: KRIISHSHARMA

include/linux/string.h

@@ -371,6 +371,10 @@ static inline void memzero_explicit(void *s, size_t count)
  * kbasename - return the last part of a pathname.
  *
  * @path: path to extract the filename from.
+ *
+ * Returns:
+ * Pointer to the filename portion inside @path. If no '/' exists,
+ * returns @path unchanged.
  */
 static inline const char *kbasename(const char *path)
 {
@@ -556,6 +560,9 @@ static __always_inline size_t str_has_prefix(const char *str, const char *prefix
  * strstarts - does @str start with @prefix?
  * @str: string to examine
  * @prefix: prefix to look for.
+ *
+ * Returns:
+ * True if @str begins with @prefix. False in all other cases.
  */
 static inline bool strstarts(const char *str, const char *prefix)
 {