Add HttpClient.BaseAddress Documentation

[scottoffen]

Summary

Documented the trailing slash requirement on HttpClient.BaseAddress when combining it with a relative route.

• Added identical <remarks> blocks to all four UsingRoute overloads (string and Uri, generic and non-generic) explaining that BaseAddress must end with a trailing slash for a relative route to be appended correctly, with a worked example and a note that a leading slash on the route does not fix it.
• Added identical <remarks> blocks to both UsingBase overloads clarifying that they don't accept or set a route, so the trailing slash requirement doesn't apply to them directly.
• Added a danger admonition to the HttpRequestBuilder docs page explaining the same behavior, with a link to RFC 3986 section 5.

No behavior changed. This is expected Uri combination behavior per RFC 3986, not a bug in FluentHttpClient, so the fix is documentation rather than code.

Related issue

Closes #58

Type

• Bug fix
• Feature
• Docs
• Other: ___

Checklist

• Descriptive title and clear description
• Tests added/updated (xUnit + Shouldly + Moq if needed)
• Docs updated (if user-facing behavior changed)
• No cosmetic/whitespace-only changes
• Follows the contribution guidelines and C# conventions, with readability as a priority

Notes

No tests added since no behavior changed, this is a documentation-only PR. Not a breaking change.