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/logn

@@ -0,0 +1,68 @@
+NAME
+    logn -  base n logarithm
+
+SYNOPSIS
+    logn(x, n [,eps])
+
+TYPES
+    x		nonzero real or complex
+    n		nonzero real or complex
+    eps		nonzero real, defaults to epsilon()
+
+    return	real or complex
+
+DESCRIPTION
+    Approximate the base n logarithm function of x by a multiple of
+    epsilon, the error having absolute value less than 0.75 * eps.
+
+    The base, n, must not be 0 nor 1.
+
+    If y is a positive integer, log(x, n^-y) will usually be correct
+    to the y-th decimal place.
+
+EXAMPLE
+    ; print logn(15.625, 2.5), logn(15.625, 2.5, 1e-25)
+    ~ 3.00000000000000000001 3
+
+    ; print logn(127, 1/13), log(23209, sqrt(3)), logn(2^17-19, 17)
+    ~ -1.88860925162778125111 6 ~ 4.15900804831225415076
+
+    ; print logn(-1, 1i), logn(2+3i, 3i+2), logn(2+3i, 3i)
+    2 1 ~ 0.80360095345990217753 - ~ 0.25441159318835790311i
+
+    ; print logn(22+3i, 3i, 1e-50)
+    ~ 0.98489914201047045408 - ~ 1.28484657882287682702i
+
+LIMITS
+    x != 0
+    n != 0 && n != 1
+    eps > 0
+
+LINK LIBRARY
+    NUMBER *qlogn(NUMBER *x, NUMBER *n, NUMBER *eps)
+    COMPLEX *c_logn(COMPLEX *x, COMPLEX *n, NUMBER *eps)
+
+SEE ALSO
+    ilog, ilogn, ilog10, ilog2, ln, log, log2
+
+## 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/