And clean-up / standardize existing docs of same.

Continuation of #193752.

Changes:

extensions/markdown-language-features/package.nls.json

1. Changed order of documented variables to match extensions/markdown-language-features/src/languageFeatures/copyFiles/copyFiles.ts.
2. Consistent documentation of all variables, using /Users/me/myProject/docs/README.md as the example Markdown document.

Before:

• ${documentFileName} — The full filename of the Markdown document, for example: readme.md.
• ${documentBaseName} — The basename of Markdown document, for example: readme.
• ${documentExtName} — The extension of the Markdown document, for example: md.
• ${documentDirName} — The name of the Markdown document's parent directory.
• ${documentWorkspaceFolder} — The workspace folder for the Markdown document, for example: /Users/me/myProject. This is the same as ${documentDirName} if the file is not part of a workspace.
• ${fileName} — The file name of the dropped file, for example: image.png.

After:

• ${documentDirName} — Absolute parent directory path of the Markdown document, e.g. /Users/me/myProject/docs.
• ${documentRelativeDirName} — Relative parent directory path of the Markdown document, e.g. docs. This is the same as ${documentDirName} if the file is not part of a workspace.
• ${documentFileName} — The full filename of the Markdown document, e.g. README.md.
• ${documentBaseName} — The basename of the Markdown document, e.g. README.
• ${documentExtName} — The extension of the Markdown document, e.g. md.
• ${documentFilePath} — Absolute path of the Markdown document, e.g. /Users/me/myProject/docs/README.md.
• ${documentRelativeFilePath} — Relative path of the Markdown document, e.g. docs/README.md. This is the same as ${documentFilePath} if the file is not part of a workspace.
• ${documentWorkspaceFolder} — The workspace folder for the Markdown document, e.g. /Users/me/myProject. This is the same as ${documentDirName} if the file is not part of a workspace.
• ${fileName} — The file name of the dropped file, e.g. image.png.
• ${fileExtName} — The extension of the dropped file, e.g. png.

extensions/markdown-language-features/src/languageFeatures/copyFiles/copyFiles.ts

Updated inline comments to match the new variable descriptions I added to extensions/markdown-language-features/package.nls.json. I caught a minor issue (albeit unlikely to trip anyone up) -- the comment after documentRelativeDirName was // Absolute parent directory path (copied unchanged from line above) -- figured I might as well just sync them up while I was at it.