Make __wasi_args_get and __wasi_environ_get docs more explicit. (#128)

This commit updates the documentation for `__wasi_args_get` and
`__wasi_environ_get` to make it explicit that it is the caller's responsibility
to allocate a terminating NULL pointer, if necessary.

Fixes #27.

Author: peterhuene

phases/ephemeral/docs/wasi_ephemeral_preview.md

@@ -117,18 +117,22 @@ modules so that embedders need not implement all of it.
 
 Read command-line argument data.
 
-The sizes of the buffers should match that returned by [`__wasi_args_sizes_get()`](#args_sizes_get).
-
 Inputs:
 
 - <a href="#args_get.argv" name="args_get.argv"></a><code>char \*\*<strong>argv</strong></code>
 
     A pointer to a buffer to write the argument pointers.
 
+    The count of elements for the buffer should be at least the value of `argc` returned by [`__wasi_args_sizes_get()`](#args_sizes_get).
+
+    The caller of `__wasi_args_get()` is responsible for allocating an additional element for a terminating NULL pointer, if necessary.
+
 - <a href="#args_get.argv_buf" name="args_get.argv_buf"></a><code>char \*<strong>argv\_buf</strong></code>
 
     A pointer to a buffer to write the argument string data.
 
+    The size of this buffer should be the value of `argv_buf_size` returned by [`__wasi_args_sizes_get()`](#args_sizes_get).
+
 ### <a href="#args_sizes_get" name="args_sizes_get"></a>`__wasi_args_sizes_get()`
 
 Return command-line argument data sizes.
@@ -192,21 +196,25 @@ Outputs:
 
 Read environment variable data.
 
-The sizes of the buffers should match that returned by [`__wasi_environ_sizes_get()`](#environ_sizes_get).
-
 Inputs:
 
 - <a href="#environ_get.environ" name="environ_get.environ"></a><code>char \*\*<strong>environ</strong></code>
 
     A pointer to a buffer to write the environment variable pointers.
 
+    The count of elements for the buffer should be at least the value of `environ_count` returned by [`__wasi_environ_sizes_get()`](#environ_sizes_get).
+
+    The caller of `__wasi_environ_get()` is responsible for allocating an additional element for a terminating NULL pointer, if necessary.
+
 - <a href="#environ_get.environ_buf" name="environ_get.environ_buf"></a><code>char \*<strong>environ\_buf</strong></code>
 
     A pointer to a buffer to write the environment variable string data.
 
+    The size of this buffer should be the value of `environ_buf_size` returned by [`__wasi_environ_sizes_get()`](#environ_sizes_get).
+
 ### <a href="#environ_sizes_get" name="environ_sizes_get"></a>`__wasi_environ_sizes_get()`
 
-Return command-line argument data sizes.
+Return enviroment variable data sizes.
 
 Outputs: