Fixes in docstrings (#1063)

* Fix docstrings for the geom_density2d() and geom_density2df() functions.

* Mention fix #1062 in the future_changes.md file.

* Fix #1047: Improve documentation for parameter position.

* Add examples with hjust/vjust to the docstrings of the functions geom_text()/geom_label().

* Small fix in docstrings for the geom_text()/geom_label() functions.

* Replace single quotation marks with standard ones.

* Small fix in docstring example of the geom_label() function.

future_changes.md

@@ -15,6 +15,8 @@
 - Support arrow() in geom_spoke() [[#986](https://github.com/JetBrains/lets-plot/issues/986)].
 - Support geom_spoke() in geom_livemap() [[#988](https://github.com/JetBrains/lets-plot/issues/988)].
 - arrow on curve sometimes looks weird [[#1041](https://github.com/JetBrains/lets-plot/issues/1041)].
+- Improve documentation for parameter `position` [[#1047](https://github.com/JetBrains/lets-plot/issues/1047)].
 - Livemap: `vjust` implemented incorrectly [[#1051](https://github.com/JetBrains/lets-plot/issues/1051)].
-- Missing marginal gridlines.
-- Add tooltips for geom_curve() [[#1053](https://github.com/JetBrains/lets-plot/issues/1053)].
+- Add tooltips for `geom_curve()` [[#1053](https://github.com/JetBrains/lets-plot/issues/1053)].
+- `geom_density2d`: the doc missing some 'computed' variables [[#1062](https://github.com/JetBrains/lets-plot/issues/1062)].
+- Missing marginal gridlines.

python-package/lets_plot/plot/core.py

@@ -95,8 +95,9 @@ def layer(geom=None, stat=None, data=None, mapping=None, position=None, **kwargs
  Aesthetic mappings describe the way that variables in the data are
  mapped to plot "aesthetics".
  position : str or `FeatureSpec`
- Position adjustment, either as a string ('identity', 'stack', 'dodge', ...),
- or the result of a call to a position adjustment function.
+ Position adjustment.
+ Either a position adjustment name: 'dodge', 'dodgev', 'jitter', 'nudge', 'jitterdodge', 'fill',
+ 'stack' or 'identity', or the result of calling a position adjustment function (e.g., position_dodge() etc.).
  kwargs:
  Other arguments passed on to layer. These are often aesthetics settings, used to set an aesthetic to a fixed
  value, like color = "red", fill = "blue", size = 3 or shape = 21. They may also be parameters to the