GTest root directory is inconsistent

### Page link:

https://bazel.build/rules/lib/builtins/runfiles

### Problem description (include actual vs expected text, if applicable):

If I have a directory with this structure

```
opencv_hello
testdata
starrry_night.jpg
```
then in gtest I can get the file with this

```
Mat mat1 =
cv::imread(runfiles->Rlocation("opencv_hello/testdata/starry_night.jpg");
```
Given the fact that I add "//testdata" into "data" in BUILD

However, if I had a testdata one level below root:

```
testdata
starrry_night.jpg
```

Then I need to have "_main":

```
Mat mat1 =
cv::imread(runfiles->Rlocation("_main/testdata/starry_night.jpg");
```

This seems inconsistent. In one case I need an upper level of directory or "_main"

### Where do you see this issue? (include link to specific section of the page, if applicable)

Every time I have directory that is directly below the root as I mentioned earlier.

### Any other information you'd like to share?

Maybe it needs an update in documentation since I don't remember seeing "_main". Ideally it would be nice to just have "testdata/file.jpg" rather than "_main" or whatever is above that.

Start with the runfiles documentation at https://bazel.build/rules/lib/builtins/runfiles and reproduce both directory layouts using the BUILD data entry and the shown GTest/C++ lookup. Compare the documented path rules with the observed need for _main, then update the page to explain the root-directory behavior and the expected testdata path.

Written by the indexing model from the issue text.