Clarify docs on GN rebase_path.

tools/gn/function_rebase_path.cc

Index: tools/gn/function_rebase_path.cc
diff --git a/tools/gn/function_rebase_path.cc b/tools/gn/function_rebase_path.cc
index 6661d8112b7480fff5fc0f3d8b478da038d9d174..ef5ef40f984046cab216970f62b9eaaac27ea21c 100644
--- a/tools/gn/function_rebase_path.cc
+++ b/tools/gn/function_rebase_path.cc
@@ -144,7 +144,7 @@ const char kRebasePath_Help[] =
     "  relative to the source root, so can't also generate source-absolute\n"
     "  paths without more special-cases.\n"
     "\n"
-    "Arguments:\n"
+    "Arguments\n"
     "\n"
     "  input\n"
     "      A string or list of strings representing file or directory names\n"
@@ -175,6 +175,12 @@ const char kRebasePath_Help[] =
     "  names will be converted to be relative to the requested output\n"
     "  System-absolute paths will be unchanged.\n"
     "\n"
+    "  Whether an output path will end in a slash will match whether the\n"
+    "  corresponding input path ends in a slash. It will return \".\" or\n"
+    "  \"./\" (depending on whether the input ends in a slash) to avoid\n"
+    "  returning empty strings. This means if you want a root path\n"
+    "  (\"//\" or \"/\") not ending in a slash, you can add a dot (\"//.\").\n"
+    "\n"
     "Example\n"
     "\n"
     "  # Convert a file in the current directory to be relative to the build\n"