add help in advance for new log2 and logn builtins

NOTE: The `log2(x [,eps])` and `logn(x, n [,eps])`
builtin functions are NOT yet implemented.

help/log2

@@ -0,0 +1,64 @@
+NAME
+    log2 -  base 10 logarithm
+
+SYNOPSIS
+    log2(x [,eps])
+
+TYPES
+    x		nonzero real or complex
+    eps		nonzero real, defaults to epsilon()
+
+    return	real or complex
+
+DESCRIPTION
+    Approximate the base 2 logarithm function of x by a multiple of
+    epsilon, the error having absolute value less than 0.75 * eps.
+
+    If y is a positive integer, log(x, 2^-y) will usually be correct
+    to the y-th decimal place.
+
+    When x if a power of 2, log2(x) will return an integer regardless
+    of the value of eps or epsilon().
+
+EXAMPLE
+    ; print log2(2), log2(4), log2(1024), log2(2^500)
+    1 2 10 500
+
+    ; print log(127), log(23209), log(2^17-19)
+    ~ 6.98868468677216585326 ~ 14.50239674255407864468 ~ 16.99979085393743521984
+
+    ; print log(2+3i, 1e-5)
+    ~ 1.85020558320709803073 + ~ 1.41786049195700786266i
+
+LIMITS
+    x != 0
+    eps > 0
+
+LINK LIBRARY
+    NUMBER *qlog2(NUMBER *x, NUMBER *eps)
+    COMPLEX *c_log2(COMPLEX *x, NUMBER *eps)
+
+SEE ALSO
+    ilog, ilogn, ilog10, ilog2, ln, log, logn
+
+## Copyright (C) 2023  Landon Curt Noll
+##
+## Calc is open software; you can redistribute it and/or modify it under
+## the terms of the version 2.1 of the GNU Lesser General Public License
+## as published by the Free Software Foundation.
+##
+## Calc is distributed in the hope that it will be useful, but WITHOUT
+## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+## or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU Lesser General
+## Public License for more details.
+##
+## A copy of version 2.1 of the GNU Lesser General Public License is
+## distributed with calc under the filename COPYING-LGPL.  You should have
+## received a copy with calc; if not, write to Free Software Foundation, Inc.
+## 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+##
+## Under source code control:	2023/08/19 09:14:04
+## File existed as early as:	2023
+##
+## chongo <was here> /\oo/\	http://www.isthe.com/chongo/
+## Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/