Update README

Author: mbostock

README.md

@@ -580,9 +580,9 @@ Faceting can be explicitly enabled or disabled on a mark with the *mark*.**facet
 * *exclude* - draw the subset of the mark’s data *not* in the current facet
 * null (or false) - repeat this mark’s data across all facets (i.e., no faceting)
 
-When top-level faceting is used, the default *auto* setting is equivalent to *include* when mark data is strictly equal to facet data; otherwise it is equivalent to null. When the *include* or *exclude* facet mode is chosen, the mark data must be parallel to the top-level facet data: the mark data must have the same length and order as the top-level facet data. If the data are not parallel, then the wrong data may be shown in each facet. The default *auto* therefore requires strict equality (`===`) for safety, and using the facet data as mark data is recommended when using the *exclude* facet mode. (To construct parallel data safely, consider using [*array*.map](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map) on the facet data.)
+When top-level faceting is used, the default *auto* setting is equivalent to *include* when the mark data is strictly equal to the top-level facet data; otherwise it is equivalent to null. When the *include* or *exclude* facet mode is chosen, the mark data must be parallel to the top-level facet data: the data must have the same length and order. If the data are not parallel, then the wrong data may be shown in each facet. The default *auto* therefore requires strict equality (`===`) for safety, and using the facet data as mark data is recommended when using the *exclude* facet mode. (To construct parallel data safely, consider using [*array*.map](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map) on the facet data.)
 
-When mark-level faceting is used, the default *auto* setting is equivalent to *include*: the mark will be faceted if a *mark*.**fx** or *mark*.**fy** channel (or both) is specified. The null or false option will disable faceting, while *exclude* draws the subset of the mark’s data *not* in the current facet.
+When mark-level faceting is used, the default *auto* setting is equivalent to *include*: the mark will be faceted if either the *mark*.**fx** or *mark*.**fy** channel option (or both) is specified. The null or false option will disable faceting, while *exclude* draws the subset of the mark’s data *not* in the current facet.
 
 ## Legends