Improve a few doc pages

[PaulWessel]

Joaquim was unhappy with how some options were documented in spectrum1d, gmt2kml, and grdimage. I could not find the problem in grd2kml.

[joa-quim]

Need to fix also the end of -N description that ends exactly like this

appearing where %d is placed, (4) give no arguments to turn symbol labeling off; line segments will still be named. Also note that

[PaulWessel]

Please be specific where this is since searching for this text in all *.c files returns no matches. What file are we talking about here? Your original message said grd2kml but now you say gmt2kml, but in either case I cannot find what you are listing.

[joa-quim]

It's man page of gmt2kml -N option (https://gmt.soest.hawaii.edu/doc/latest/gmt2kml.html#n)

-N[t|col |name_template|name]
    By default, if segment headers contain a -L“label string” then we use that for the name of the KML feature (polygon, line segment or set of symbols). Default names for these segments are
“Line %d” and “Point Set %d”, depending on the feature, where %d is a sequence number of
line segments within a file. Each point within a line segment will be named after the line
segment plus a sequence number. Default is simply “Point %d”. Alternatively, select one of
these options: (1) append col to supply individual symbol labels as the string formatted from
the col data column, (2) append t to let individual symbol labels be the trailing text of each
record (3) append a string that may include %d or a similar integer format to assign unique
name IDs for each feature, with the segment number (for lines and polygons) or point number
(symbols) appearing where %d is placed, (4) give no arguments to turn symbol labeling off; line
segments will still be named. Also note that 

[joa-quim]

The gmt2kml -N option ends exactly like this

...appearing where %d is placed, (4) give no arguments to turn symbol labeling off; line segments will still be named. Also note that