Explain the logic behind negative slice indices.

src/core/array.c

@@ -273,7 +273,8 @@ static const JanetReg array_cfuns[] = {
              "Takes a slice of array or tuple from start to end. The range is half open, "
              "[start, end). Indexes can also be negative, indicating indexing from the end of the "
              "end of the array. By default, start is 0 and end is the length of the array. "
-             "Returns a new array.")
+             "Note that index -1 is synonymous with index (length arrtup) to allow a full "
+             "negative slice range. Returns a new array.")
     },
     {
         "array/concat", cfun_array_concat,

src/core/string.c

@@ -525,7 +525,8 @@ static const JanetReg string_cfuns[] = {
              "Returns a substring from a byte sequence. The substring is from "
              "index start inclusive to index end exclusive. All indexing "
              "is from 0. 'start' and 'end' can also be negative to indicate indexing "
-             "from the end of the string.")
+             "from the end of the string. Note that index -1 is synonymous with "
+             "index (length bytes) to allow a full negative slice range. ")
     },
     {
         "string/repeat", cfun_string_repeat,

src/core/tuple.c

@@ -143,7 +143,10 @@ static const JanetReg tuple_cfuns[] = {
         JDOC("(tuple/slice arrtup [,start=0 [,end=(length arrtup)]])\n\n"
              "Take a sub sequence of an array or tuple from index start "
              "inclusive to index end exclusive. If start or end are not provided, "
-             "they default to 0 and the length of arrtup respectively."
+             "they default to 0 and the length of arrtup respectively. "
+             "'start' and 'end' can also be negative to indicate indexing "
+             "from the end of the input. Note that index -1 is synonymous with "
+             "index '(length arrtup)' to allow a full negative slice range. "
              "Returns the new tuple.")
     },
     {