Improve documentation

The Cosmo API documentation page is pretty good now
https://justine.lol/cosmopolitan/documentation.html

libc/nexgen32e/bsrll.c

@@ -19,6 +19,23 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/nexgen32e/bsr.h"
 
+/**
+ * Returns binary logarithm of 𝑥.
+ *
+ *                           ctz(𝑥)         31^clz(𝑥)   clz(𝑥)
+ *       uint32 𝑥   bsf(𝑥) tzcnt(𝑥)   ffs(𝑥)   bsr(𝑥) lzcnt(𝑥)
+ *     0x00000000      wut       32        0      wut       32
+ *     0x00000001        0        0        1        0       31
+ *     0x80000001        0        0        1       31        0
+ *     0x80000000       31       31       32       31        0
+ *     0x00000010        4        4        5        4       27
+ *     0x08000010        4        4        5       27        4
+ *     0x08000000       27       27       28       27        4
+ *     0xffffffff        0        0        1       31        0
+ *
+ * @param x is a 64-bit integer
+ * @return number in range 0..63 or undefined if 𝑥 is 0
+ */
 int(bsrll)(long long x) {
   return bsrll(x);
 }