Update docstring to explain case sensitivity

pdfsearch/pdfsearch.py

@@ -43,6 +43,16 @@ def search_string(pattern, string, match_object, context_length):
 def search_pdf(pdf_name, search_patterns, context_length):
     """Search for text strings inside a pdf.
 
+    Search patterns should be lowercase, unless case-sensitivity is important.
+
+    'ph' will match:
+       - photograph
+       - PHANTOM
+       - pH
+
+    'pH' will match:
+       - pH
+
     Args:
         pdf_name:        path to pdf file
         search_patterns: list of re search patterns