[gnumeric] Doc fixes

[Morten Welinder <mortenw src gnome org>]

commit 6c00075696ff7656f81185300a910b013061f21a
Author: Morten Welinder <terra gnome org>
Date:   Mon Nov 4 20:28:26 2019 -0500

    Doc fixes

 src/mathfunc.c | 16 +++++++++++++++-
 src/sf-gamma.c |  6 +++---
 2 files changed, 18 insertions(+), 4 deletions(-)
---
diff --git a/src/mathfunc.c b/src/mathfunc.c
index 7ab0b9c57..f51b72261 100644
--- a/src/mathfunc.c
+++ b/src/mathfunc.c
@@ -5041,8 +5041,22 @@ gnm_lambert_w (gnm_float x, int k)
 
        if (gnm_isnan (x) || x < -one_over_e)
                return gnm_nan;
-       else if (x == -one_over_e)
+       else if (x == -one_over_e) {
+               // This is technically wrong.  The mathematically right
+               // value of 1/e is 0.367879441171442321595524...
+               // which rounds to 0.367879441171442334024277... as a double.
+               // Note, that the rounding went up.  In other words, when we
+               // observe "x == -one_over_e" then x is already less that the
+               // mathematically right value of -1/e and we ought to return
+               // NaN.  That is, however, a somewhat useless behaviour so
+               // we return -1 instead.
+               //
+               // NOTE 1: The analysis might be different for long double.
+               // NOTE 2: The analysis assumes that 1/e when computed as double
+               //         division (based on rounded e) produces the right
+               //         value.  It does, see goffice's test/constants
                return -1;
+       }
 
        if (k == 0) {
                if (x == gnm_pinf)
diff --git a/src/sf-gamma.c b/src/sf-gamma.c
index a2d260e72..45751af45 100644
--- a/src/sf-gamma.c
+++ b/src/sf-gamma.c
@@ -1633,7 +1633,7 @@ gnm_digamma_series_1 (gnm_float x)
 {
        static const gnm_float ctr = 3414350731.0 / 4294967296.0; // ~ x0-2/3
        // Taylor series data for digamma(x)*x around ctr
-       // (Multiplying by x eliminates the pole at 0 and inproves convergence)
+       // (Multiplying by x eliminates the pole at 0 and improves convergence)
 
        // There are more terms here than will be used in practice
        static const gnm_float c[] = {
@@ -1702,7 +1702,7 @@ static gnm_float
 gnm_digamma_series_2 (gnm_float x, gnm_float dx)
 {
        // Taylor series data for digamma(x)*x around x0.
-       // (Multiplying by x eliminates the pole at 0 and inproves convergence)
+       // (Multiplying by x eliminates the pole at 0 and improves convergence)
 
        // There are more terms here than will be used in practice
        static const gnm_float c[] = {
@@ -1772,7 +1772,7 @@ gnm_digamma_series_3 (gnm_float x)
        static const gnm_float ctr = 9140973792.0 / 4294967296.0; // ~ x0+2/3
 
        // Taylor series data for digamma(x)*x*(x+1) around ctr.
-       // (Multiplying by x eliminates the pole at 0 and inproves convergence;
+       // (Multiplying by x eliminates the pole at 0 and improves convergence;
        // multiplying by x+1 removes trouble caused by the pole at -1.)
        //
        // There are more terms here than will be used in practice