mainloop: Add some helpful comments

Added a couple helpful comments that save someone
from reading the code and thinking about it to know
what it does.

Change-Id: Ieac770e99fbd75a42be6803ee7959aba6ad6e96a
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/mainloop.cpp b/mainloop.cpp
index c93beb8..62467fd 100644
--- a/mainloop.cpp
+++ b/mainloop.cpp
@@ -450,12 +450,15 @@
         _isOCC = true;
     }
 
+    // Strip off any trailing slashes.
     std::string p = path;
     while (!p.empty() && p.back() == '/')
     {
         p.pop_back();
     }
 
+    // Given the furthest right /, set instance to
+    // the basename, and hwmonRoot to the leading path.
     auto n = p.rfind('/');
     if (n != std::string::npos)
     {