Improve <system_error> functions page

[Rageking8]

Improvements (first commit):

• Add descriptions for is_error_code_enum_v and is_error_condition_enum_v helper variable templates
• Remove indent and inline keyword for is_error_code_enum_v and is_error_condition_enum_v syntax
  • https://github.com/microsoft/STL/blob/1f6e5b16ec02216665624c1e762f3732605cf2b4/stl/inc/system_error#L48-L49
  • https://github.com/microsoft/STL/blob/1f6e5b16ec02216665624c1e762f3732605cf2b4/stl/inc/system_error#L57-L58
  • https://eel.is/c++draft/system.error.syn
• Fix wrong mention of "error code" in error parameter description of make_error_condition
• Update description for system_category (not sure what "low-level system overflows" is, hence changed it to be inlined with the description of generic_category)

Cleanups (second commit):

• Edit metadata
• Simplify redundant page links
• Remove 2 empty Remarks section

(Will propagate the new descriptions in bulk to https://learn.microsoft.com/en-us/cpp/standard-library/system-error?view=msvc-170 in a future PR)

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[ttorble]

@TylerMSFT

Can you review the proposed changes?

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[TylerMSFT]

An inline constexpr. Things that make you go hmmm...
Thank you for the cleanup :-)
Also, thank you for the ../standardlibrary cleanup. I see that sort of thing a lot and its nice for it to go away.

[TylerMSFT]

#sign-off