Update `HttpHeaders.Add` exception docs

[MihaZupan]

• Being more specific about what a valid header name format is.
• Fix misleading/outdated comment about new line chars in header values.

[learn-build-service-prod[bot]]

Learn Build status updates of commit 9bae508:

:x: Validation status: errors

Please follow instructions here which may help to resolve issue.

File	Status	Preview URL	Details
[]()	:x:Error		Details
xml/System.Net.Http.Headers/HttpHeaders.xml	:x:Error		Details

• Line 0, Column 0: [Error: ECMA2Yaml_File_LoadFailed] Failed to load 1 files, aborting...

xml/System.Net.Http.Headers/HttpHeaders.xml

• Line 0, Column 0: [Error: ECMA2Yaml_InternalError]

  Intenal Several Error: System.Xml.XmlException: An error occurred while parsing EntityName. Line 165, position 193.
  at System.Xml.XmlTextReaderImpl.Throw(Exception e)
  at System.Xml.XmlTextReaderImpl.Throw(String res, Int32 lineNo, Int32 linePos)
  at System.Xml.XmlTextReaderImpl.HandleEntityReference(Boolean isInAttributeValue, EntityExpandType expandType, Int32& charRefEndPos)
  at System.Xml.XmlTextReaderImpl.ParseText(Int32& startPos, Int32& endPos, Int32& outOrChars)
  at System.Xml.XmlTextReaderImpl.FinishPartialValue()
  at System.Xml.XmlTextReaderImpl.get_Value()
  at System.Xml.Linq.XContainer.ContentReader.ReadContentFrom(XContainer rootContainer, XmlReader r)
  at System.Xml.Linq.XContainer.ReadContentFrom(XmlReader r)
  at System.Xml.Linq.XDocument.Load(XmlReader reader, LoadOptions options)
  at System.Xml.Linq.XDocument.Parse(String text, LoadOptions options)
  at ECMA2Yaml.ECMALoader.LoadType(FileItem typeFile)
  at ECMA2Yaml.ECMALoader.LoadTypes(String basePath, Namespace ns)

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[learn-build-service-prod[bot]]

Learn Build status updates of commit 3e2307d:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
xml/System.Net.Http.Headers/HttpHeaders.xml	:white_check_mark:Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel