Repository: nilearn/nilearn

Issue:
Add notes about ICBM template and the relationship with fMRIprep default template
<!--Describe your proposed enhancement in detail.-->
It is not clear that the default template used by fMRPrep (https://github.com/templateflow/tpl-MNI152NLin2009cAsym) is the ICBM template.
This need to be make clear in the doc string.

<!--List any pages that would be impacted by the enhancement.-->
### Affected pages

https://nilearn.github.io/modules/generated/nilearn.datasets.fetch_icbm152_2009.html#nilearn.datasets.fetch_icbm152_2009

Edit: wrong template flow link


Discussion:
After reading a bit more in detail, I found that the version in nilearn is not exactly the same as fMRIprep:
| | fmriprep | nilearn|
|-|-|-|
|Symmetric?| Asymmetric | Symmetric|
|Version?| c | a|

I also found out there's redundancy in the structural image.
The MNI template data in `nilearn.datasets` are the same one fetching from osf in `nilearn.datasets.fetch_icbm152_2009`.
Good news is nilearn is using the latest iteration of MNI152, not the old one shipped in FSL, but not the  most up to date one
Do we want to consider update / trim down the current data shipped in nilearn?
This is not going to have a huge impact when it comes to plotting, but for tissue segmentation it might be. I am not an expert on this one, please feel free to chip in.

Here's probably the cleanest summary of all the different template derived from MNI152 dataset: 
http://nist.mni.mcgill.ca/atlases/
We have changed the template less than a year ago, so I think it is not a huge issue. But we should warn users ('the ICB template image will be changed in the  release R from XXX to YYY').

Resolving patch:
diff --git a/nilearn/_utils/docs.py b/nilearn/_utils/docs.py
--- a/nilearn/_utils/docs.py
+++ b/nilearn/_utils/docs.py
@@ -731,6 +731,15 @@ def custom_function(vertices):
 
 """
 
+# templateflow
+docdict["templateflow"] = """
+    The default template of :term:`fMRIPrep` is the asymmetrical ICBM152 2009,
+    release c (MNI152NLin2009cSAsym). The NiLearn template is asymmetrical
+    ICBM152 2009, release a. If you wish to use the exact same release as
+    :term:`fMRIPrep`, please refer to TemplateFlow
+    (https://www.templateflow.org/).
+"""
+
 docdict_indented = {}
 
 
diff --git a/nilearn/datasets/struct.py b/nilearn/datasets/struct.py
--- a/nilearn/datasets/struct.py
+++ b/nilearn/datasets/struct.py
@@ -46,6 +46,8 @@
 def fetch_icbm152_2009(data_dir=None, url=None, resume=True, verbose=1):
     """Download and load the ICBM152 template (dated 2009).
 
+    %(templateflow)s
+
     For more information, see :footcite:`FONOV2011313`,
     :footcite:`Fonov2009`, and :footcite:`Collins1999algorithm`.
 
@@ -113,6 +115,13 @@ def fetch_icbm152_2009(data_dir=None, url=None, resume=True, verbose=1):
     The original download URL is
     http://www.bic.mni.mcgill.ca/~vfonov/icbm/2009/mni_icbm152_nlin_sym_09a_nifti.zip
 
+    TemplateFlow repository for ICBM152 2009
+
+    Symmetric: https://github.com/templateflow/tpl-MNI152NLin2009cSym
+
+    Asymmetric: https://github.com/templateflow/tpl-MNI152NLin2009cSAsym
+
+
     """
     if url is None:
         # The URL can be retrieved from the nilearn account on OSF (Open
@@ -176,6 +185,9 @@ def load_mni152_template(resolution=None):
 
     See Also
     --------
+    nilearn.datasets.fetch_icbm152_2009: for details regarding the difference
+        between NiLearn and :term:`fMRIPrep` ICBM152 template.
+
     nilearn.datasets.load_mni152_gm_template : for details about version of the
         MNI152 grey-matter template.
 
@@ -497,6 +509,8 @@ def fetch_icbm152_brain_gm_mask(data_dir=None, threshold=0.2, resume=True,
                                 n_iter=2, verbose=1):
     """Downloads ICBM152 template first, then loads the 'gm' mask.
 
+     %(templateflow)s
+
     .. versionadded:: 0.2.5
 
     Parameters


Test patch: