Filter() description and code snippet clarity and consistency

[kevinmckinley]

Changes

Reword the description and code snippets for Filter() in OpenTelemetry.Instrumentation.* READMEs.

Grammar and code syntax updated to be consistent between AspNet, AspNetCore, and Http.
Where components were inconsistent, the following decisions were made. These can easily be changed if desired.

• Lambda parameter syntax. Using of () for single params eg. (x) => {}
• AspNetInstrumentationOptions parameter name options instead of opt
• Single statement lambdas do not use {} eg. (x) => x.foo() instead of (x) => { x.foo(); }
• README code referenced as "code snippet" instead of "example"
• Tab spacing for code snippets.

Issue driving change: #1186
Previous related PRs: #1615

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Design discussion issue Instrumentation Filter possible confusion with return type. #1186
• Changes in public API reviewed