Take some of @armfazh's suggestions for common conventions

These changes are mostly for clarity, but there are a few things we had
underspecified:

* The length of the output of the little-endian and big-endian encoders
  was not specified.

* How the length of the little-endian and big-endian decoders was
  interpreted was not specified.

* the length of the output of `additive_secret_shares()` was ambiguous.

draft-irtf-cfrg-vdaf.md

@@ -767,7 +767,7 @@ Type hints are used to define input and output types:
 * `Self` represents the containing class of the method definition in which it
   appears.
 
-* `Sequence[T]` is either a list or tuple.
+* `Sequence[T]` is either a list or tuple of values of type `T`.
 
 This document defines several byte-string constants. When comprised of printable
 ASCII characters, they are written as Python 3 byte-string literals (e.g.,
@@ -784,8 +784,8 @@ a particular encoding of that quantity as a byte string.
 Some common functionalities:
 
 * `additive_secret_share(x: list[F], num_shares: int, field: type[F]) ->
-  list[list[F]]` takes a vector `x` of field elements and returns multiple
-  vectors of the same length, such that they all add up to the input vector.
+  list[list[F]]` takes a vector `x` of field elements and returns `num_shares`
+  vectors of length `len(x)` such that they all add up to the input vector.
   Note that this function is not used normatively in this document.
 
 * `byte(x: int) -> bytes` returns the representation of the integer `x` in range
@@ -799,15 +799,18 @@ Some common functionalities:
 * `concat(parts: list[bytes]) -> bytes` returns the concatenation of the input
   byte strings, i.e., `parts[0] + ... + parts[len(parts)-1]`.
 
-* `from_be_bytes(encoded: bytes) -> int` computes the inverse of
-  `to_be_bytes()`.
+* `from_be_bytes(encoded: bytes) -> int` decodes a big-endian byte string,
+  i.e., returns the integer `x` for which `to_be_bytes(x, len(encoded)) ==
+  encoded`.
 
-* `from_le_bytes(encoded: bytes) -> int` computes the inverse of
-  `to_le_bytes()`.
+* `from_le_bytes(encoded: bytes) -> int` decodes a little-endian byte string,
+  i.e., returns the integer `x` for which `to_le_bytes(x, len(encoded)) ==
+  encoded`.
 
 * `front(len: int, x: list[Any]) -> tuple[list[Any], list[Any]]` splits `x`
   into two vectors, where the first vector is made up of the first `len`
-  elements of `x`. I.e., `(x[:len], x[len:])`.
+  elements of `x` and the second is made up of the remaining elements. This
+  function is equivalent to `(x[:len], x[len:])`.
 
 * `gen_rand(len: int) -> bytes` returns a byte array of the requested length
   (`len`) generated by a cryptographically secure pseudorandom number generator
@@ -823,14 +826,16 @@ Some common functionalities:
   output values.
 
 * `to_be_bytes(x: int, len: int) -> bytes` converts an integer `x` whose value
-  is in the range `[0, 2^(8*len))` to big-endian bytes.
+  is in the range `[0, 2^(8*len))` to a big-endian, `len`-byte string.
 
 * `to_le_bytes(x: int, len: int) -> bytes` converts an integer `x` whose value
-  is in the range `[0, 2^(8*len))` to little-endian bytes.
+  is in the range `[0, 2^(8*len))` to a little-endian, `len`-byte string.
 
 * `poly_eval(field: type[F], p: list[F], x: F) -> F` returns the result of
   evaluating the polynomial, `p(x)`. The coefficients of polynomials are stored
-  in lists in ascending order of degree, starting with the constant coefficient.
+  in lists in ascending order of degree, starting with the constant
+  coefficient. The `field` parameter is the class object for `F` and is used by
+  the implementation to construct field elements. (See {{field}}.)
 
 * `poly_interp(field: type[F], inputs: list[F], outputs: list[F]) -> list[F]`
   returns the coefficients of the lowest degree polynomial `p` for which
@@ -847,8 +852,8 @@ Some common functionalities:
 * `xor(left: bytes, right: bytes) -> bytes` returns the bitwise XOR of `left`
   and `right`. An exception is raised if the inputs are not the same length.
 
-* `zeros(len: int) -> bytes` returns an array of zero bytes of the requested
-  length (`len`).
+* `zeros(len: int) -> bytes` returns an array of bytes of the requested
+  length (`len`). Each element of the array is set to zero.
 
 # Overview