[gnumeric] LAMBERTW: Improve docs.

[Morten Welinder <mortenw src gnome org>]

commit 9e8b308ac4d17b89d6a4df4146f052c2c7b97e7b
Author: Morten Welinder <terra gnome org>
Date:   Mon May 4 19:23:54 2020 -0400

    LAMBERTW: Improve docs.
    
    See #485, item 1.

 plugins/fn-math/functions.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/plugins/fn-math/functions.c b/plugins/fn-math/functions.c
index c4e08c434..ca6f01698 100644
--- a/plugins/fn-math/functions.c
+++ b/plugins/fn-math/functions.c
@@ -1275,8 +1275,9 @@ static GnmFuncHelp const help_lambertw[] = {
        { GNM_FUNC_HELP_ARG, F_("k:branch")},
        { GNM_FUNC_HELP_NOTE, F_("@{k} defaults to 0, the principal branch.") },
        { GNM_FUNC_HELP_NOTE, F_("@{k} must be either 0 or -1.") },
+       { GNM_FUNC_HELP_DESCRIPTION, F_("The Lambert W function is the inverse function of x=W*exp(W).  There 
are two (real-valued) branches: k=0 which maps [-1/e;inf) onto [-1,inf); and k=-1 which maps [-1/e;0) unto 
(-inf;1].") },
        { GNM_FUNC_HELP_EXAMPLES, "=LAMBERTW(3)" },
-       { GNM_FUNC_HELP_EXAMPLES, "=LAMBERTW(3,-1)" },
+       { GNM_FUNC_HELP_EXAMPLES, "=LAMBERTW(-1/4,-1)" },
        { GNM_FUNC_HELP_SEEALSO, "EXP"},
        { GNM_FUNC_HELP_END}
 };