DOC: Add summary lines to plot types.

IMHO it's meaningful to give a one-sentence summary on the individual
pages, e.g. at
https://matplotlib.org/stable/plot_types/basic/plot.html#sphx-glr-plot-types-basic-plot-py

> Plot y versus x as lines and/or markers.
>
> See `plot`.

is more helpful than just

> See `plot`.

Users can already decide if that's the correct function for them and
whether it's worth following the link. Also, the first sentence is used
in the tooltip on the index page, where the current "See ..." is not
helpful.

I believe it's ok to just copy the text. Automatic insertion or check
for consistency are not worth the added effort. First, these summary
lines change rarely. Second, even if we improve the method summary and
forget to udpate here, nothing really bad happens; we've maybe a
slightly less optimal description here, but that's still way better than
no description at all.

Semi-OT: I changed the barbs summary from

> Plot a 2D field of barbs.

to

> Plot a 2D field of wind barbs.

because, AFAIK these are only used for wind, and that makes the context
more clear. People outside of metrology may not know what barbs are
so giving that extra context is reasonable.

Author: timhoffm

galleries/plot_types/arrays/barbs.py

@@ -2,6 +2,7 @@
 =================
 barbs(X, Y, U, V)
 =================
+Plot a 2D field of wind barbs.
 
 See `~matplotlib.axes.Axes.barbs`.
 """

galleries/plot_types/arrays/contour.py

@@ -2,6 +2,7 @@
 ================
 contour(X, Y, Z)
 ================
+Plot contour lines.
 
 See `~matplotlib.axes.Axes.contour`.
 """

galleries/plot_types/arrays/contourf.py

@@ -2,6 +2,7 @@
 =================
 contourf(X, Y, Z)
 =================
+Plot filled contours.
 
 See `~matplotlib.axes.Axes.contourf`.
 """

galleries/plot_types/arrays/imshow.py

@@ -2,6 +2,7 @@
 =========
 imshow(Z)
 =========
+Display data as an image, i.e., on a 2D regular raster.
 
 See `~matplotlib.axes.Axes.imshow`.
 """

galleries/plot_types/arrays/pcolormesh.py

@@ -2,6 +2,7 @@
 ===================
 pcolormesh(X, Y, Z)
 ===================
+Create a pseudocolor plot with a non-regular rectangular grid.
 
 `~.axes.Axes.pcolormesh` is more flexible than `~.axes.Axes.imshow` in that
 the x and y vectors need not be equally spaced (indeed they can be skewed).

galleries/plot_types/arrays/quiver.py

@@ -2,6 +2,7 @@
 ==================
 quiver(X, Y, U, V)
 ==================
+Plot a 2D field of arrows.
 
 See `~matplotlib.axes.Axes.quiver`.
 """

galleries/plot_types/arrays/streamplot.py

@@ -2,6 +2,7 @@
 ======================
 streamplot(X, Y, U, V)
 ======================
+Draw streamlines of a vector flow.
 
 See `~matplotlib.axes.Axes.streamplot`.
 """

galleries/plot_types/basic/fill_between.py

@@ -2,6 +2,7 @@
 =======================
 fill_between(x, y1, y2)
 =======================
+Fill the area between two horizontal curves.
 
 See `~matplotlib.axes.Axes.fill_between`.
 """

galleries/plot_types/basic/plot.py

@@ -2,6 +2,7 @@
 ==========
 plot(x, y)
 ==========
+Plot y versus x as lines and/or markers.
 
 See `~matplotlib.axes.Axes.plot`.
 """

galleries/plot_types/basic/scatter_plot.py

@@ -2,6 +2,7 @@
 =============
 scatter(x, y)
 =============
+A scatter plot of y vs. x with varying marker size and/or color.
 
 See `~matplotlib.axes.Axes.scatter`.
 """