[PXEXT] Update documentation

Add documentation for the FILE_EXEC and FILE_CHECK_API extension
calls, and update the documentation for the FILE_READ call.

src/doc/pxe_extensions

@@ -125,8 +125,9 @@ Output:		PXENV_EXIT_SUCCESS or PXENV_EXIT_FAILURE must be
 		This API function is non-blocking.  PXENV_EXIT_SUCCESS
 		and PXENV_STATUS_SUCCESS is returned if a data block
 		has been transferred into the caller's buffer.
-		PXENV_EXIT_FAILURE and PXENV_STATUS_FAILURE is
-		returned if no data is available to transfer.
+		PXENV_EXIT_FAILURE and PXENV_STATUS_TFTP_OPEN is
+		returned if no data is available to transfer; any
+		other status code reflects an error.
 
 Description:	Read from a previously opened file.
 
@@ -191,3 +192,88 @@ Returned from API service:
 FileSize:	Size of the file in bytes.
 
 Status:		See PXENV_STATUS_xxx constants.
+
+
+
+
+FILE EXEC
+
+Op-Code:	PXENV_FILE_EXEC (00e5h)
+
+Input:		Far pointer to a t_PXENV_FILE_EXEC parameter
+		structure that has been initialized by the caller.
+
+Output:		PXENV_EXIT_SUCCESS or PXENV_EXIT_FAILURE must be
+		returned in AX.  The Status field in the parameter
+		structure must be set to one of the values represented
+		by the PXENV_STATUS_xxx constants.
+
+Description:	Execute a gPXE command.
+
+typedef struct s_PXENV_FILE_EXEC {
+        PXENV_STATUS_t Status;
+        SEGOFF16_t Command;
+} t_PXENV_FILE_EXEC;
+
+
+Set before calling API service:
+
+Command:	Command to execute.  Null terminated.
+
+
+Returned from API service:
+
+Status:		See PXENV_STATUS_xxx constants.
+
+
+
+
+FILE API CHECK
+
+Op-Code:	PXENV_FILE_API_CHECK (00e6h)
+
+Input:		Far pointer to a t_PXENV_FILE_CHECK_API parameter
+		structure that has been initialized by the caller.
+
+		On entry, the Magic field should contain the number
+		0x91d447b2 or the call will fail.
+
+Output:		PXENV_EXIT_SUCCESS or PXENV_EXIT_FAILURE must be
+		returned in AX.  The Status field in the parameter
+		structure must be set to one of the values represented
+		by the PXENV_STATUS_xxx constants.
+
+		If this API is present and the Magic field contains the
+		proper value on entry, AX will contain PXENV_EXIT_SUCCESS,
+		the Status field PXENV_STATUS_SUCCESS, and the Magic field
+		the number 0xe9c17b20.  Any other combination should be
+		considered a failure.
+
+Description:	Detect presence of this API.
+
+
+typedef struct s_PXENV_FILE_CHECK_API {
+	PXENV_STATUS Status;
+	UINT16 Size;
+	UINT32 Magic;
+	UINT32 Provider;
+	UINT32 APIMask;
+	UINT32 Flags;
+} t_PXENV_FILE_CHECK_API;
+
+Set before calling API service:
+
+Size:		Set to sizeof(t_PXENV_FILE_CHECK_API) (20).
+Magic:		Set to 0x91d447b2.
+
+
+Returned from API service:
+
+Size:		Set to the number of bytes filled in (20).
+Magic:		Set to 0xe9c17b20.
+Provider:	Set to 0x45585067 ("gPXE").  Another implementation of this
+		API can use another value, e.g. to indicate a different
+		command set supported by FILE EXEC.
+APIMask:	Bitmask of supported API functions (one bit for each function
+		in the range 00e0h to 00ffh).
+Flags:		Set to zero, reserved for future use.