Document that PhysicalFilesWatcher can watch paths that don't exist

[svick]

Also fixes formatting of PhysicalFilesProvider.Watch() parameter:

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Pull request overview

Updates XML documentation for Microsoft.Extensions.FileProviders.Physical to clarify that watchers/tokens can be created even when the target root path or watched entries don’t exist yet, and cleans up the Watch(string filter) parameter docs to render examples correctly.

Changes:

• Document that PhysicalFilesWatcher can watch a root directory that does not exist yet.
• Clarify that Watch / CreateFileChangeToken patterns don’t require the target files/directories to exist at call time.
• Improve XML doc formatting for PhysicalFileProvider.Watch(string filter) examples.

File summaries

File	Description
src/libraries/Microsoft.Extensions.FileProviders.Physical/src/PhysicalFilesWatcher.cs	Expands XML docs to clarify non-existent root and non-existent watched entries are supported at token creation time.
src/libraries/Microsoft.Extensions.FileProviders.Physical/src/PhysicalFileProvider.cs	Updates constructor and Watch parameter documentation to reflect non-existent roots/entries and improves example formatting.

Review details

Suppressed comments (2)

src/libraries/Microsoft.Extensions.FileProviders.Physical/src/PhysicalFilesWatcher.cs:86

• XML doc comments in this library generally avoid contractions. Consider using "is not" here for consistency and to keep the API docs more formal.

        /// <param name="root">The root directory for the watcher. The directory isn't required to exist.</param>

src/libraries/Microsoft.Extensions.FileProviders.Physical/src/PhysicalFileProvider.cs:50

• XML doc comments in this library generally avoid contractions. Consider using "is not" here for consistency and to keep the API docs more formal.

        /// <param name="root">The root directory. This should be an absolute path. The directory isn't required to exist.</param>

• Files reviewed: 2/2 changed files
• Comments generated: 4
• Review effort level: Lite