Document the ialloc function abbreviations.

In the jemalloc_internal_inlines files, we have a lot of somewhat terse function
names.  This commit adds some documentation to aid in translation.

include/jemalloc/internal/cache_bin.h

@@ -17,7 +17,7 @@
 /*
  * The count of the number of cached allocations in a bin.  We make this signed
  * so that negative numbers can encode "invalid" states (e.g. a low water mark
- * for a bin that has never been filled).
+ * of -1 for a cache that has been depleted).
  */
 typedef int32_t cache_bin_sz_t;
 

include/jemalloc/internal/jemalloc_internal_inlines_c.h

@@ -5,6 +5,24 @@
 #include "jemalloc/internal/sz.h"
 #include "jemalloc/internal/witness.h"
 
+/*
+ * Translating the names of the 'i' functions:
+ *   Abbreviations used in the first part of the function name (before
+ *   alloc/dalloc) describe what that function accomplishes:
+ *     a: arena (query)
+ *     s: size (query, or sized deallocation)
+ *     e: extent (query)
+ *     p: aligned (allocates)
+ *     vs: size (query, without knowing that the pointer is into the heap)
+ *     r: rallocx implementation
+ *     x: xallocx implementation
+ *   Abbreviations used in the second part of the function name (after
+ *   alloc/dalloc) describe the arguments it takes
+ *     z: whether to return zeroed memory
+ *     t: accepts a tcache_t * parameter
+ *     m: accepts an arena_t * parameter
+ */
+
 JEMALLOC_ALWAYS_INLINE arena_t *
 iaalloc(tsdn_t *tsdn, const void *ptr) {
 	assert(ptr != NULL);