Netbeans supports javadoc comments in c code, but sometimes it can't find a description that Netbeans even generated itself:
For example, I have a function and I want to add a javadoc comment.
I write /***/, press ENTER and Netbeans generate something like
/**
 *
 * @param x
 * @return OK
 */

Then I add a description to this comment:
/**
 * Some function with 1 parameter
 * @param x
 * @return OK
 */

But only for some functions Netbeans will show this description in autocompletion window. For others it will show "Documentation not found".

Also If there was a javadoc comment for a define, for example
/**javadoc to MY_DEFINE1*/
#define MY_DEFINE1 1
#define MY_DEFINE2 2
#define MY_DEFINE3 3

then in autocompletion screen we will see "comment to MY_DEFINE1" for all three defines instead of "Documentation not found" for defines 2 and 3.