DOC: Add docstrings to filter view functions (networkx#7086)

* Update filters.py

* Update filters.py

* Apply black.

---------

Co-authored-by: Ross Barnowski <rossbar@caltech.edu>

networkx/classes/filters.py

@@ -18,36 +18,44 @@
 
 
 def no_filter(*items):
+    """Returns a filter function that always evaluates to True."""
     return True
 
 
 def hide_nodes(nodes):
+    """Returns a filter function that hides specific nodes."""
     nodes = set(nodes)
     return lambda node: node not in nodes
 
 
 def hide_diedges(edges):
+    """Returns a filter function that hides specific directed edges."""
     edges = {(u, v) for u, v in edges}
     return lambda u, v: (u, v) not in edges
 
 
 def hide_edges(edges):
+    """Returns a filter function that hides specific undirected edges."""
     alledges = set(edges) | {(v, u) for (u, v) in edges}
     return lambda u, v: (u, v) not in alledges
 
 
 def hide_multidiedges(edges):
+    """Returns a filter function that hides specific multi-directed edges."""
     edges = {(u, v, k) for u, v, k in edges}
     return lambda u, v, k: (u, v, k) not in edges
 
 
 def hide_multiedges(edges):
+    """Returns a filter function that hides specific multi-undirected edges."""
     alledges = set(edges) | {(v, u, k) for (u, v, k) in edges}
     return lambda u, v, k: (u, v, k) not in alledges
 
 
 # write show_nodes as a class to make SubGraph pickleable
 class show_nodes:
+    """Filter class to show specific nodes."""
+
     def __init__(self, nodes):
         self.nodes = set(nodes)
 
@@ -56,20 +64,24 @@ def __call__(self, node):
 
 
 def show_diedges(edges):
+    """Returns a filter function that shows specific directed edges."""
     edges = {(u, v) for u, v in edges}
     return lambda u, v: (u, v) in edges
 
 
 def show_edges(edges):
+    """Returns a filter function that shows specific undirected edges."""
     alledges = set(edges) | {(v, u) for (u, v) in edges}
     return lambda u, v: (u, v) in alledges
 
 
 def show_multidiedges(edges):
+    """Returns a filter function that shows specific multi-directed edges."""
     edges = {(u, v, k) for u, v, k in edges}
     return lambda u, v, k: (u, v, k) in edges
 
 
 def show_multiedges(edges):
+    """Returns a filter function that shows specific multi-undirected edges."""
     alledges = set(edges) | {(v, u, k) for (u, v, k) in edges}
     return lambda u, v, k: (u, v, k) in alledges