DOC: impedances may be obtained for BrainVision (#12861)

Co-authored-by: Clemens Brunner <[email protected]>

mne/io/brainvision/brainvision.py

@@ -63,6 +63,12 @@ class RawBrainVision(BaseRaw):
 
     Notes
     -----
+    If the BrainVision header file contains impedance measurements, these may be
+    accessed using ``raw.impedances`` after reading using this function. However,
+    this attribute will NOT be available after a save and re-load of the data.
+    That is, it is only available when reading data directly from the BrainVision
+    header file.
+
     BrainVision markers consist of a type and a description (in addition to other fields
     like onset and duration). In contrast, annotations in MNE only have a description.
     Therefore, a BrainVision marker of type "Stimulus" and description "S  1" will be
@@ -977,6 +983,12 @@ def read_raw_brainvision(
 
     Notes
     -----
+    If the BrainVision header file contains impedance measurements, these may be
+    accessed using ``raw.impedances`` after reading using this function. However,
+    this attribute will NOT be available after a save and re-load of the data.
+    That is, it is only available when reading data directly from the BrainVision
+    header file.
+
     BrainVision markers consist of a type and a description (in addition to other fields
     like onset and duration). In contrast, annotations in MNE only have a description.
     Therefore, a BrainVision marker of type "Stimulus" and description "S  1" will be