clarify cell target in doc

docs/cggnn.md

@@ -54,7 +54,7 @@ Note that with this configuration file, parameters can't be eliminated to use th
 * `test_data_percent`: Percentage of data to use as the test set. (Training percentage is implicit.)
 * `disable_channels`: If true, disable the use of individual channel information in the graph. (Only named phenotypes, a.k.a. ``composite phenotypes", would appear.)
 * `disable_phenotypes`: If true, disable the use of phenotype information in the graph. (Only individual channels would appear.)
-* `cells_per_roi_target`: An intended target number of cells per slide, used with the median cell density across all slides during determination of the ROI size.
+* `cells_per_roi_target`: Target number of cells per region of interest (ROI). Used with the median cell density across all slides to choose what (one) ROI size will be chosen for the study. (The actual number of cells per ROI will vary as cell density isn't uniform across slides.)
 * `target_name`: The name of a column in the dataframe whose true values indicate cells to be used when building ROIs. Can be set to `none` to use all cells in the sample.
 * `in_ram`: If true, store the data in RAM.
 * `batch_size`: Batch size to use during training.